• *The discussion of the creation, fabrication, or modification of airgun moderators is prohibited. The discussion of any "adapters" used to convert an airgun moderator to a firearm silencer will result in immediate termination of the account.*

How to slug a barrel? 🤔

Jump to latest Follow Reply
Matthias,

Apart from "how", the question is "why" slug a barrel? If you know why, then the best technique, and what you do with the info gained might become clearer.

For many, slugging is the salad before the main course - which might be lapping; or shooting as is. What is on your menu? We don't know which airgun you mean to slug, or the caliber. For pellet shooting, or slugs? If the latter, what is the twist rate of the barrel?
 
  • Like
Reactions: markhooper
Upvote 0
⭐ Friends,

this thread shows again what a great community we are on the forums! 😊

Thanks for all your very helpful answers. Great! 👍🏼

To start with the most recent question of Peter — the WHY SLUG a barrel?
➠ Answer: To BUY SLUGS that fit the barrel. 😃
Because I think I finally have the power and maybe the barrel to shoot them with reasonable precision.

I also have a slug collection with a ridiculously(!) wide variety. 🤦🏻‍♂️
Somehow I feel the need to justify having spent so much time and money to assemble that lead array....

Matthias
 
  • Like
Reactions: markhooper, tibor and subscriber
Upvote 0
Thanks, Matthias

Slug fit is important. The next aspect of equal importance is the twist rate of the barrel, so you know the maximum weight (length) of slug that will be stable and shoot well. If your barrel has a twist optimized for pellets, it may not shoot anything but short light slugs well. Twist rate and caliber interact. The larger the caliber, the lower the twist rate required for a given projectile length (often limited by the magazine depth)

Of course, you can buy or make slugs of different lengths and weights; then try them out. Just don't buy a boatload of slugs that are too long to shoot well, and be stuck with them.

The next aspect is the power of your PCP. It needs to be able to drive slugs fast enough for them to have an advantage over pellets. If expansion in flesh is important, then that typically sets a minimum impact velocity. If you don't have the power to drive heavy slugs fast enough, then you are stuck with shorter lighter ones. In which case, not having a fast enough twist barrel for long heavy slugs may be moot.

Anyway, slugging is a first step; and you have to start somewhere.

I assume you are talking about your new .25 PCP?

Merry Christmas
 
Last edited:
  • Like
Reactions: markhooper
Upvote 1
🔶 You were asking about barrels, guns, and power.
Here:

The guns I want to slug are:
● RTI Prophet I • .22cal • CZ 400mm barrel • Choked • 1:17.7"(?) twist rate • 30FPE

● RTI Prophet II • .25cal • RPB 510mm barrel • Lightly choked • 1:20" twist rate ▪ 60FPE

● Diana Skyhawk • .22cal • LW aftermarket 500mm barrel • Choked • 1:17.5" twist rate ▪ 30FPE




🔶 OK, I'm trying to write a step-by-step summary of your excellent suggestions. Please, correct me, or answer my questions as indicated below: 👍🏼


1️⃣ Get a wooden dowel, a bit longer than the barrel — a thin stick (smaller than the caliber of the barrel). They are usually sold for woodworking or in craft supply stores. A rigid barrel cleaning rod might work, too, but can damage the barrel. Brass is preferred as it is softer than the steel of the barrel. Best is a carbon fiber rod.

2️⃣ Remove the barrel from the gun, with all the precautions necessary for removal and reinstallation.

3️⃣ Remove any silencers or shrouds from the barrel.

4️⃣ Take a slug. It is preferable that the slug is slightly "too large" for the barrel. This could be accomplished by hitting it lightly and straight down with a hammer while it stands on a solid surface.

5️⃣ Using the dowel, push a slug from the breech to the muzzle.
Some lubricant might make this easier (clean up afterwards).
If the barrel is choked, do not push it all the way through, but stop before the choke, and push the slug back out the breech.
It is best to use a shorter dowel at first, as they easily break off.

6️⃣ Get a caliper that is able to accurately measure (not just display) lenghts of 0.0001" (0.00254mm) (±10% precision of 0.001").
Or get a micrometer.

I checked, and the seller declares that my calipers don't provide that accuracy 🤔 — as they only are accurate to 0.001" (meaning they could be off by ±50%, so that 0.0015" and 0.0005" would read the same 0.001"). ➠❓Is this correct reasoning?



7️⃣ Count the grooves of the barrel.
● If they are an even number, measure the size of the slug with a caliper after pushing it through the barrel.
● If the grooves are an uneven number, the caliper would not give an accurate reading (as one of the measuring brackets would be measuring an indentation from a barrel land instead of the full diameter of the slug).
Therefore, use a thin flexible piece of metal. ➠ ❓What kind — where would I get that from?
Measure the thickness of the metal.
Wrap it tightly around the slug once; this will give an even surface for measuring with the caliper. Measure the diameter (slug plus the metal wrap).
From the diameter measurement deduct the thickness of the metal wrap *twice*. This is called the groove diameter of the barrel — the diameter from groove to groove (the indentations in the barrel, not the lands [ridges]). ➠ ❓Is that correct?


8️⃣ The slug diameter that most likely will be better than others is the barrel inside diameter PLUS a certain amount that is not agreed upon:
I see people saying between 0.0005" and 0.001" (0.0254mm and 0.0127mm), others simply 0.001", and others yet 0.003".
➠ So this seems to be a point of disagreement...? Maybe? ❓


Matthias
 
  • Like
Reactions: markhooper
Upvote 1
Matthias,

Most of what you have looks good, except the wire for measuring the grooves: You want the equivalent of a small diameter gauge pin that you place in a groove and then measure over that to the barrel groove diameter opposite the land diameter (groove in slug with pin reaching its bottom). The shank of a small diameter twist drill may work; or a small dowel pin. Using a pin for such slug groove depth measurement is a bit like measuring a thread's effective diameter, rather than its OD.

Wrapping anything tightly around a soft lead slug will distort the slug, so you need gentle contact to avoid that.

If your 30 FPE .22 PCPs cannot be tuned up, then you can only shoot 15 grain slugs at 950 FPS, or 20 grain slugs at a little under 850 FPS. This is if the slug friction is the same as for pellets, which is not the case. So, unless you can tune the airguns a little hotter, you may not achieve 30 FPE with slugs.

The .25 sounds like it has potential for light slug use also, as tuned. If it can be tuned hotter, better still. The .22s and .25 should have adequate twist for slugs that can be driven at useful speeds. You could try heavier slugs and see what can be fired at 800 FPS, although from what I understand, small bore PCP slugs like being driven fast to group consistently. If we were talking about a .357 or .45, then 800 FPS should be "fast enough".

At some velocity, slugs will have loopy trajectories and perhaps reduce the advantage over pellets to the point of making them costly alternatives, without clear advantage. That said, even slugs of the same weight as pellets should retain their velocity better than pellets, due to a slug's higher BC. It is just that for long range shooting a loopy trajectory is less useful - unless you know the range and how much drop to compensate for. Then, a lack of expansion with pointy slugs in animals at lower velocity becomes the next hurdle.

Back to slugging the barrel; to start the slug fully into the barrel bore, it is likely to require tapping the back of the short dowel with a light hammer. This is because light hammer blows can easily generate very high forces, that are also easy to control. Leaning on a stick with you full weight is likely to be very unpleasant when the slug suddenly breaks free and moves into the barrel. Once the slug is started, lighter taps can be used to drive it on the longer dowel, or possibly just force with your hand.

Do not clamp the barrel tightly in a vice. Not even with padded jaws - unless the jaws perfectly match the radius of the barrel OD. A thin walled barrel will spring oval by a good fraction of 0.001" in a tight vice with flat jaws. As you care about good measurements, it is better to stand the barrel upright on a padded surface, so the slugging resisting force is carried by the muzzle face. Just not on the inside of the crown. You can use a vice or a clamp to constrain the barrel, if you do not clamp it tight enough to resist the slugging force.

Avoiding clamping a barrel tight enough spring it out of round is even more important if you lap the bore: You could lap an oval into the bore, that shows after the clamping force is relieved. Making wooden jaws for the vice that have a half cylinder groove at a radius that matches the barrel should reduce this risk. My goal is not to scare you but to make you think about the process. If the barrel wall is thick, I am less concerned about springing it out of round.
 
  • Like
Reactions: Ezana4CE and markhooper
Upvote 0
What is the best slug diameter may not be as large as groove diameter, or over it, if your barrels have chokes that reduce the bore diameter by several 0.001". One has to start somewhere, but I would not go larger on the slug OD than 0.001" over the groove diameter at the choke. Ultimately what shoots best trumps all theory.
 
  • Like
Reactions: markhooper
Upvote 0
TheShooter (aka Seeker) said:
When you perform barrel lapping you need to have so called barrel lead lap. You pour melted lead into the bore and that is your mold which produces a negative sculpture of your bore. Then you apply some polishing compound on the lap and you are ready to make your bore as consistent as possible. However, it is a time consuming process and it needs some practice to be done correctly. The funnel is there to make it easier to pour the lead into the bore.

View attachment 418710
You may ask what it has in common with slugging of the bore. When you slug a bore you may notice some loose and tight spots. Lapping is a process to achieve a very consistent bore. So slugging and lapping are correlated. You slug then lap then slug and lap again. It takes a lot of time and patience but it allows you to achieve a perfect bore.
Click to expand...
wow, learn something more each time.. I hadn't heard of pouring a slug in the barrel and then lapping with it, but it totally makes sense.
Mark
 
  • Like
Reactions: TheShooter (aka Seeker)
Upvote 0
DesertSilver said:
You won't be happy with those slugs in your avenger. It has a very tight .215 groove diameter and the 30 grain BBT can't be sized down that far. The 25 grain slugs work much better after being sized down to .215.
https://noebulletmolds.com/site/shop/bullet-moulds/224/218-25-rf-d6/218-25-rf-d6-rg2-2-cavity/

I have a liberty which uses the same barrel as the Aspen, freedom, and avenger. Not sure about the X but I have a suspicion that it's the same barrel also. My liberty did real well with the 25 grain slug.
Click to expand...
that's why I was saying that I could get a new barrel, preferably lothar Walther barrel blank and machine it to fit..
I took have a liberty that I really like.. I've had it 8 years without any trouble.. but it has a very slow leak.. goes down 1000 psi per week and stops where I think regulator pressure is set and well it's stayed there for 3 weeks after the first refill..
I don't understand why it stops leaking at 2000 psi, I assume regulator pressure.. I've worked on a lot of things but I don't understand how the regulator works, along with the atmosphere vent.. I think it must be in the regulator leaking at the atmosphere vent, only because I had a shop air regulator for painting that did leak down my spray gun and tank to whatever settings it was on..
since you have a liberty, do you have any idea if my guess on the regulator is correct?
any trouble shooting ideas? also I would love to see a exploded view of the regulator and if possible the air flow path, and how it is leaking..
much thanks for any help with it.
Mark
 
Upvote 0
subscriber said:
Thanks, Matthias

Slug fit is important. The next aspect of equal importance is the twist rate of the barrel, so you know the maximum weight (length) of slug that will be stable and shoot well. If your barrel has a twist optimized for pellets, it may not shoot anything but short light slugs well. Twist rate and caliber interact. The larger the caliber, the lower the twist rate required for a given projectile length (often limited by the magazine depth)

Of course, you can buy or make slugs of different lengths and weights; then try them out. Just don't buy a boatload of slugs that are too long to shoot well, and be stuck with them.

The next aspect is the power of your PCP. It needs to be able to drive slugs fast enough for them to have an advantage over pellets. If expansion in flesh is important, then that typically sets a minimum impact velocity. If you don't have the power to drive heavy slugs fast enough, then you are stuck with shorter lighter ones. In which case, not having a fast enough twist barrel for long heavy slugs may be moot.

Anyway, slugging is a first step; and you have to start somewhere.

I assume you are talking about your new .25 PCP?

Merry Christmas
Click to expand...
I have a question, you mentioned twist rate and if it is not fast enough to stabilize a heavy slug.. just wondering what happens if you have a fast twist rate and then shoot lighter slugs.. are they over stable? or tumble? how do you determine what twist rate is best for the bullet weight..
I wish my friend was still alive, he was target shooting with me and he was trying different barrel twist rate and different bullets in his 223 but I don't remember what he found that works.. I'm guessing heavy bullets and fast twist and light bullets and slow twist rate.. but I don't really remember.. mostly I remember that the 223 was less accurate when he was testing than any gun I had.. I could outshoot him on the rifle range with my pistol.. but it was not a competition, just a couple guys having fun.. sure do miss him, he taught me mechanic, gas and diesel and transmission.. taught me welding and machine work.. couldn't ask for a better friend, he even loaned me a lathe to learn on at my house for around a year until I found a good used one.. and I was like 14 years old at the time..worked together later on a 70 ft bridge with him and I had already done so much more welding that he asked me to do all the overhead and vertical welds.. it is funny that I can actually weld vertical better than flat.. he was a great friend..
I don't know, I might be wrong but if we all did things like that it would definitely be a better place..
also Yes this forum has a lot of great helpful people to share with and learn from.
Mark
 
Upvote 0
Too slow a twist rate and groups open, just before holes in paper targets elongate, just before the slugs are obviously tumbling. So there is a progression of trouble, rather than a switch as you try slower twist rates, or longer slugs.

A faster barrel twist rate than required causes less trouble with slugs. Some would argue there is no downside. With pellets, too fast a barrel twist, combined with too high a muzzle velocity tends to cause the pellets to spiral after about 50 yards. You can restart the videos to see the whole story, but I have them queued at a point of interest:


 
  • Like
Reactions: markhooper
Upvote 0
markhooper said:
that's why I was saying that I could get a new barrel, preferably lothar Walther barrel blank and machine it to fit..
I took have a liberty that I really like.. I've had it 8 years without any trouble.. but it has a very slow leak.. goes down 1000 psi per week and stops where I think regulator pressure is set and well it's stayed there for 3 weeks after the first refill..
I don't understand why it stops leaking at 2000 psi, I assume regulator pressure.. I've worked on a lot of things but I don't understand how the regulator works, along with the atmosphere vent.. I think it must be in the regulator leaking at the atmosphere vent, only because I had a shop air regulator for painting that did leak down my spray gun and tank to whatever settings it was on..
since you have a liberty, do you have any idea if my guess on the regulator is correct?
any trouble shooting ideas? also I would love to see a exploded view of the regulator and if possible the air flow path, and how it is leaking..
much thanks for any help with it.
Mark
Click to expand...
I replaced all of the Orings with the Capt Oring kit and it holds very good. The regulator and the poppet are the 2 main leak points I've found. Right now I've got a leak from the gage that's installed on the plenum that Duy made. It's the second one that has gone bad so I'm probably going to just plug it as I've got the regulator set where I want it. Regulators are easy to understand.
Maybe this will help.
www.huma-air.com

Function of a regulator

Function of a regulator - Huma-Air - Order easily and secure ✔️High quality products ✔️ Worldwide shipping
www.huma-air.com www.huma-air.com
 
  • Like
Reactions: markhooper
Upvote 0
DesertSilver said:
I replaced all of the Orings with the Capt Oring kit and it holds very good. The regulator and the poppet are the 2 main leak points I've found. Right now I've got a leak from the gage that's installed on the plenum that Duy made. It's the second one that has gone bad so I'm probably going to just plug it as I've got the regulator set where I want it. Regulators are easy to understand.
Maybe this will help.
www.huma-air.com

Function of a regulator

Function of a regulator - Huma-Air - Order easily and secure ✔️High quality products ✔️ Worldwide shipping
www.huma-air.com www.huma-air.com
Click to expand...
yes AOA told me that I get the captain o ring kit.. does it have all the seals? how about the poppet that you had leaking?
thank you all for all the wealth of information..
Mark
 
Upvote 0
subscriber said:
Too slow a twist rate and groups open, just before holes in paper targets elongate, just before the slugs are obviously tumbling. So there is a progression of trouble, rather than a switch as you try slower twist rates, or longer slugs.

A faster barrel twist rate than required causes less trouble with slugs. Some would argue there is no downside. With pellets, too fast a barrel twist, combined with too high a muzzle velocity tends to cause the pellets to spiral after about 50 yards. You can restart the videos to see the whole story, but I have them queued at a point of interest:


Click to expand...
thank you for the videos.. I will have to take time and watch them a few times.. because I always like to learn new things..
Mark
 
  • Like
Reactions: subscriber
Upvote 0

markhooper​


Back to your question of over stabilization, Palma shooter using .308 Win rifles that sport 30" long barrels, prefer a barrel twist rate that is just enough to keep the bullets stable to the point the velocity has dropped to 1200 FPS. They have found that provides most consistent grouping for them at the ranges they care about. They like to use a "Palma" 155 grain bullet and a 1:12 barrel twist, shooting the bullets at very close to 3000 FPS from their long barrels. They want the bullet velocity to be at least 1300 FPS at 1000 yards, so they are above the transonic velocity range where twist requirements would be higher. More Palma detail here:
So, the above is an example of deliberately using the slowest twist that a system can get away with. Then there is the 9 mm pistol caliber that typically uses 1:10 twist. Why that was chosen is anyone's guess, but no one questions it for being optimal, except bullseye or other competition pistol shooters. They often use special barrels with 1:20 or 1:32 twist. More detail here:

KKM 9mm barrel 1:20 vs 1:32 twist rate

I know the KKM 9mm 1:32 barrel is really accurate with 115 gr Sierra and XTP bullets, but does anyone have any experience with it shooting factory 115gr FMJ? I'm having a 9mm 1911 longslide built for target shooting but I do not reload. I know I can purchase these 115 gr match loads from Atlanta ...
forums.brianenos.com

The bottom line is that barrel twist is not enough to determine if a given projectile will be stable. The barrel length, or more precisely the projectile velocity reached is the other half of the equation.

There are two safe zones for projectile stability: Supersonic all the way to the target; and subsonic all the way to the target. Blunt nosed projectiles are more forgiving of starting supersonic and then dropping subsonic before reaching the target such as used by the .45-70 or magnum handgun calibers; but in general it makes optimizing the barrel twist more difficult.

The 9 x 19 mm pistol caliber has a muzzle velocity that is just supersonic at the muzzle and then drops subsonic, depending on the bullet weight, barrel length and range to target. So, the idea that we are dealing with absolute numbers when it comes to optimal projectile stability is clearly not the case.

The slugs used by PCP rifles are very much shaped like pistol bullets, else the connection with the above info could seem moot.

There are several rifling twist calculators out there that offer a good starting point for the barrel twist rate specific calibers require with specific projectile length, shape and mass. Boat tail projectiles require a faster spin rate, as to spitzers of a given weight.

Because airguns usually shoot subsonic projectiles (yes, they can shoot light projectiles supersonic, but not very well), if you detune them from a muzzle velocity of perhaps 1000 FPS to 600 FPS, projectile stability is not degraded at the lower velocity, but increased. This is because rifling twist requirements go up (faster twist required) as the projectile velocity is increased towards the speed of sound. This is the opposite of supersonic calibers, where if you cut the barrel short; or use light propellant charges with heavy bullets, then the barrel twist rate may need to be "faster" to create more headroom for stability.

We can predict which projectiles will probably work in a given PCP, based on the experience of other airgunners. Or we can try various slugs for ourselves to see what happens. Ultimately, the whole system needs to be in harmony to shoot well, including diameter matching, weight, shape, velocity, and twist requirements. If you talk to Nick at Nielsen Specialty Ammo, he will tell you that the exact shape of an airgun slug matters a great deal. One that looks pretty may not shoot as well as you would like. The position of the center of gravity relative to the center of pressure in flight matters a great deal. These affect the actual spin rate requirements for optimal grouping, especially as you stretch the range to 100 yards or over. Nick tests his airgun slugs at 100 yards; not just 50 for that reason.
 
Last edited:
  • Like
Reactions: markhooper
Upvote 0
markhooper said:
yes AOA told me that I get the captain o ring kit.. does it have all the seals? how about the poppet that you had leaking?
thank you all for all the wealth of information..
Mark
Click to expand...
The poppet isn't included. I've got a peek poppet that Duy aka darkchrisma made for me. Unfortunately he doesn't make parts anymore. What I did was cut the rim of the can back about 1/10 of an inch using a Dremel with a cutoff wheel with the poppet chucked into my drill press. It worked for a little while but had to be done again. You might be able to get someone to make a replacement poppet out of delron. The peek will last forever but it takes some real skills to get it to seal.
I've installed all of the mods described in his video.
 
  • Like
Reactions: markhooper
Upvote 0
subscriber said:
[What to Measure and How:]

Matthias,

Most of what you have looks good, except the wire for measuring the grooves: You want the equivalent of a small diameter gauge pin that you place in a groove and then measure over that to the barrel groove diameter opposite the land diameter (groove in slug with pin reaching its bottom). The shank of a small diameter twist drill may work; or a small dowel pin. Using a pin for such slug groove depth measurement is a bit like measuring a thread's effective diameter, rather than its OD.
Click to expand...


Peter, 😊

thank you very much for your time and effort to share all this with me (and the rest of the forum).

So, I am still confused when speaking of grooves and lands.

(a) The barrel illustrations on the interwebs mostly show firearm barrels and the ones they show have both grooves and lands that have the same width.

(b) The two barrels that I have loose without any disassembly have huge grooves, and very tiny lands — at least how my ageing eyes perceive it.
Here the important diameter seems to be the diameter from groove to groove. ❓Correct?

(c) You and others have been saying that I should measure the groove diameter of the barrel. ➔ Well, that would mean on the slug I would measure the diameter of the ridges, since the grooves of the barrel leave ridges on the slug. ❓ Right?

(d) Measuring trick (similar but reverse idea as the metal foil trick):
You are saying that when I measure the slug, I should put a metal pin on whatever I am measuring of the slug — and then subtract the pin's diameter — in order to get the barrel diameter. ❓ Correct?




subscriber said:
[Power and Velocity — Drop and Expansion of Slugs:]
If your 30 FPE .22 PCPs cannot be tuned up, then you can only shoot 15 grain slugs at 950 FPS, or 20 grain slugs at a little under 850 FPS. This is if the slug friction is the same as for pellets, which is not the case. So, unless you can tune the airguns a little hotter, you may not achieve 30 FPE with slugs.

The .25 sounds like it has potential for light slug use also, as tuned. If it can be tuned hotter, better still. The .22s and .25 should have adequate twist for slugs that can be driven at useful speeds. You could try heavier slugs and see what can be fired at 800 FPS, although from what I understand, small bore PCP slugs like being driven fast to group consistently. If we were talking about a .357 or .45, then 800 FPS should be "fast enough".

At some velocity, slugs will have loopy trajectories and perhaps reduce the advantage over pellets to the point of making them costly alternatives, without clear advantage. That said, even slugs of the same weight as pellets should retain their velocity better than pellets, due to a slug's higher BC. It is just that for long range shooting a loopy trajectory is less useful - unless you know the range and how much drop to compensate for. Then, a lack of expansion with pointy slugs in animals at lower velocity becomes the next hurdle.
Click to expand...


🔘 Power:
You are right that slugs usually develop lower power than pellets in typical PCPs.
Looking at test reports in .22cal there seems to be a very rough average of a 2FPE loss with medium weight slugs, and up to 4FPE with heavier slugs, or larger calibers.


🔘 Velocity:
I have collected over the years plenty of posts of shooters on the forums who shoot slugs at lower velocities just fine (i.e., with precision). So, I'll see what is possible with my guns.


🔘 Drop:
I have no problem with projectile drop at range and "lobbing them in" like some like to refer to it....
My range finder is pretty precise, even on a laser target as meager as a pigeon....
If I dial 6 mils or 16 mils up doesn't make much of a difference — with scopes that have a max. elevation adjustment of 25mil+ and 6mil canted rails and 25mil adjustable scope rings a lot of range is possible these days....



Cheers, 😊

Matthias
 
Last edited:
Upvote 0
Matthias,

You are correct in your understanding. Yes, talking about measuring the grooves gets confusing. The grooves in the barrel leave raised sections on the slug that you measure. If the number of lands (and grooves) is an even number, then forget the whole wire or gauge pin measurement aspect. That is only required for an uneven number of lands. (or if your measurement instrument cannot reach the bottom of the grooves made in the slug by the barrel lands; and you feel you must know the barrel land diameter - also called the bore diameter)

It does not matter how wide the lands and grooves are; except if you must know the barrel land diameter. Then you need to measure the grooves in the slug. That means you need to reach the bottom of the grooves in the slug, and if the grooves are narrow, you need a way of reaching the bottom, without contacting the side - or spanning the groove in the slug.

As you would choose or size your slugs in relation to the groove diameter of the barrel, the "lands" on the slug, or outside diameter of the slug is what matters.

You could measure the "grooves" in the slug, made by the lands in the barrel because more information is often useful, but it is not essential. Not unless you have a slug with a "bore riding" nose section. In that case you need to know the barrel land diameter to ensure that the slugs you want to shoot are slightly larger than the barrel's land diameter at the forward "bore riding" section of the slug. If the slug has only a short driving band at the rear, then its forwards "bore riding" section must never be smaller than the barrel's land diameter, or the slug will tip out of axial alignment with the barrel bore. But, the forward bearing section of a slug does not need to be at or near groove diameter to achieve this - just 0.001 or 0.002" larger than the barrel land diameter near the barrel breech. The point of bore riding front bearing slugs is to reduce friction, while still minimizing air leakage with the larger OD band at the rear. So, similar to a pellet in that regard.

Hope that helps
 
Upvote 0
subscriber said:

markhooper​


Back to your question of over stabilization, Palma shooter using .308 Win rifles that sport 30" long barrels, prefer a barrel twist rate that is just enough to keep the bullets stable to the point the velocity has dropped to 1200 FPS. They have found that provides most consistent grouping for them at the ranges they care about. They like to use a "Palma" 155 grain bullet and a 1:12 barrel twist, shooting the bullets at very close to 3000 FPS from their long barrels. They want the bullet velocity to be at least 1300 FPS at 1000 yards, so they are above the transonic velocity range where twist requirements would be higher. More Palma detail here:
So, the above is an example of deliberately using the slowest twist that a system can get away with. Then there is the 9 mm pistol caliber that typically uses 1:10 twist. Why that was chosen is anyone's guess, but no one questions it for being optimal, except bullseye or other competition pistol shooters. They often use special barrels with 1:20 or 1:32 twist. More detail here:

KKM 9mm barrel 1:20 vs 1:32 twist rate

I know the KKM 9mm 1:32 barrel is really accurate with 115 gr Sierra and XTP bullets, but does anyone have any experience with it shooting factory 115gr FMJ? I'm having a 9mm 1911 longslide built for target shooting but I do not reload. I know I can purchase these 115 gr match loads from Atlanta ...
forums.brianenos.com

The bottom line is that barrel twist is not enough to determine if a given projectile will be stable. The barrel length, or more precisely the projectile velocity reached is the other half of the equation.

There are two safe zones for projectile stability: Supersonic all the way to the target; and subsonic all the way to the target. Blunt nosed projectiles are more forgiving of starting supersonic and then dropping subsonic before reaching the target such as used by the .45-70 or magnum handgun calibers; but in general it makes optimizing the barrel twist more difficult.

The 9 x 19 mm pistol caliber has a muzzle velocity that is just supersonic at the muzzle and then drops subsonic, depending on the bullet weight, barrel length and range to target. So, the idea that we are dealing with absolute numbers when it comes to optimal projectile stability is clearly not the case.

The slugs used by PCP rifles are very much shaped like pistol bullets, else the connection with the above info could seem moot.

There are several rifling twist calculators out there that offer a good starting point for the barrel twist rate specific calibers require with specific projectile length, shape and mass. Boat tail projectiles require a faster spin rate, as to spitzers of a given weight.

Because airguns usually shoot subsonic projectiles (yes, they can shoot light projectiles supersonic, but not very well), if you detune them from a muzzle velocity of perhaps 1000 FPS to 600 FPS, projectile stability is not degraded at the lower velocity, but increased. This is because rifling twist requirements go up (faster twist required) as the projectile velocity is increased towards the speed of sound. This is the opposite of supersonic calibers, where if you cut the barrel short; or use light propellant charges with heavy bullets, then the barrel twist rate may need to be "faster" to create more headroom for stability.

We can predict which projectiles will probably work in a given PCP, based on the experience of other airgunners. Or we can try various slugs for ourselves to see what happens. Ultimately, the whole system needs to be in harmony to shoot well, including diameter matching, weight, shape, velocity, and twist requirements. If you talk to Nick at Nielsen Specialty Ammo, he will tell you that the exact shape of an airgun slug matters a great deal. One that looks pretty may not shoot as well as you would like. The position of the center of gravity relative to the center of pressure in flight matters a great deal. These affect the actual spin rate requirements for optimal grouping, especially as you stretch the range to 100 yards or over. Nick tests his airgun slugs at 100 yards; not just 50 for that reason.
Click to expand...
ok well I am on what I consider low budget.. what I want is either #1 possibly a avenger X 22, shooting the heaviest slugs that fit the magazine.. for larger varmint if I have to, but not likely.. coon to coyote size.. also target shooting long range.. 100+ yards and I thought it could be set up for the long range high power.. but then if I wanted to plink.. maybe it will be able to shoot pellets with the transfer port on low..
#2 option is the umarex gauntlet SL 30.. I know that would have plenty of power if I was forced to shoot a large varmint but I am not familiar with the difference in the size compared to 22.. in both these rifles and I don't know if the 22 is flatter than 30; or about the same or worse.. id only assume that the 30 being heavier would be less effected by wind at long distance.. but I don't think the regulator is adjustable on the umarex gauntlet 30.. although I am not so much into tuning.. id just prefer it to work out of the box
primary use is long distance target shooting, id love to be able to stretch it out to a 1/4 mile but not if it is just occasional accidental hitting targets.. I do have other rifles so I could use those for plinking and shorter targets..
last thing would be if I had to shoot a varmint.. I have had to shoot racoons during the day attacking the dog and one time me.. I assume rabies.. but that's been at least 5 years ago and last time I was shooting coyote, they were after the herd of cows I was watching.. no cows here since 2020 so unlikely.. plus we have no climb fence 6'tall..
I think I would prefer 22 avenger X if it is up to the distance shooting, just because I think if the transfer port is turned down it might possibly shoot pellets under 100 yards..I don't think I'd mind switching barrels but if I went with the avenger X, I could do a different barrel but ideally someone could tell me what regulator pressure setting is and hammer spring adjustment is and set it and leave it.. guess what I am trying to say is I have no interest in adjusting and tuning and keep playing around with different settings..
I actually have 3 PCP 22 caliber currently and they have just been shooting out of the box,, no adjustments have been done, other than cleaning the blitz and cleaning the hammer and sear area and putting in better grease for a better trigger pull.. but I do know that I can get that trigger better than it is, if I just pull the hammer and polish it and re grease.. it's got a really rough finish.
also if it takes two different barrel, I guess I can always get a barrel swap kit and then put in a lothar Walther barrel in one shroud if that's what is needed for slugs
both guns are about the same price range..
what is your opinion? I haven't shot a single varmint since 2020 so it's mostly target shooting.. not competitive.. just for fun.. I currently have a range off the back covered deck with a 3" spinner at 30 yards for pistol and 50 yards for rifle.. still working on clearing the range that we have that is longer.. I think I am around 150 yards currently but if I get finished it will be 300 yards.. planning on marking it off in 50 yards increments..for anything longer I will have to hike around on BLM behind our place and see if I can find a good, safe opening..
mark
 
Upvote 0
Mark,

I think your long range aspirations may need a bit more power than the stock Avenger delivers. If you are not up to massaging it to deliver more, perhaps this following will inspire you:

Lots of videos at link directly below:
www.altaros.cz

Altaros M24

Altaros M24 .22 - Magazine version
www.altaros.cz www.altaros.cz



1400 Yards new Airgun World Record

We used the Christmas holiday in Altaros company to set new World Record in the longest Airgun hit. The current record is 1400 yards, 1280 meters and was hit a chain wall target of size 22x18" https://youtu.be/B5ioDdftZIM This is probably the first time in modern history that this...
www.airgunnation.com www.airgunnation.com
 
Upvote 0
You must log in or register to reply here.
Top Bottom
Back
Home
Forums
Resources
Menu