What is the Ideal Twist Rate of Slugs up to .25 Caliber?

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SpiralGroove

SpiralGroove

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Oct 20, 2020
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Guys,
Have gone to the Barrel Outlet's (GS) twist calculator which suggests .177, .20 , .22 and .25 cal. slugs traveling at 950 fps have a twist rate of NO LESS 1 every 11". This seems a very fast twist rate ... much faster than the OEM RAW HM1000x, which I believe is 1 every 17.7".


thebarreloutlet.com


Twist Calculator ​


The Barrel Outlet's Barrel Twist Calculator Calculator is a tool for determining the Gyroscopic Stability (GS) of your bullet and is based on the Don Miller twist rule. (Learn More). This calculator is used to find the optimal twist rate for your bullet. This is a general information...

thebarreloutlet.com
thebarreloutlet.com

What am I missing?
 
At this point in time, is there any general repository of information anywhere that handles slugs? Watching YouTubers all say that they are all different, and just have to be tuned, but has anyone compiled any generalities yet? Like, as you mentioned @thomasair the length of the slug makes a difference. So what are the variables to making a slug work, what are they?
  • Slug Diameter
  • Slug Velocity
  • Slug Length
  • Barrel Twist Rate
  • Barrel Choking
  • Barrel Length
  • Type of rifeling (Smooth twist, button, hammer, etc.)
I'm sure that there are more, but this is a good start. Can we come up with a formula or something?
 
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I also have a question in this regard, would a Gamo Bull barrel for their break barrels, such as the Swarm, and others, I heard some Gamo Varmints also have bull barrels on them, be able to stabilize a 25+ grain all lead slug, if it had a really powerful power plant? I mean, an even more powerful one, than their 33mm compression chamber cylinder that they're running on their Gamo Magnum rifle platforms, which are delivering up to 25 ft/lbs of energy at the muzzle. Let's say they'd make a break barrel PCP with a power plant delivering 40 ft/lbs in .22 caliber, would a 1:16 twist rate bore with the groove depth that the Gamo Bull barrel has, be able so stabilize a, let's say, JSB KnockOut 25.4 grain slug?!

I reckon 28gr Norica Destroyer slugs would be traveling way too fast, because they're copper coated, or otherwise slugs that are entirely made from copper or are copper coated, but lead on the inside, would be a big no, no. What about heavier slugs than the JSB KOs, like 40gr Žans or even 58gr Altaros slugs?

So once again, a 1:16" Gamo Bull barrel/Maxxim barrel with a high powered energy plant and 25+ gr slugs?
Would this combination work out?
 
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FarmPlinker said:
Some people say unchocked barrels you can try slugs. Accuracy is a crapshoot, some do good, others are horrible. Even the most accurate have waaaaay more drop than pellets. A British airgun magazine tested the hybrid FX slugs iirc in a <12-ft.-lb rifle. Accuracy was fine, but they dropped like rocks.
Click to expand...
Just like springers, some shoot slugs well, others do horrible. The thing is, that two different rifles, of the same model, usually won't shoot slugs the same.
 
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This is from Bob Sterne:

By higher, do you mean faster (the number in inches is smaller), or a larger number (in inches), so a slower twist rate?.... It would help if you could link to "Barrel Outlet's" twist rate calculator.... Some are designed only for PB's and Supersonic, a few (very few) are intended to be used for Subsonic and Transonic, which is a different kettle of fish....

OK, so I Googled it, and it uses the "Don Miller Twist Rule", which is definitely not intended for lower velocities.... In fact, at 1200 fps and under, it gives a warning that says "Muzzle Velocity LOW!!".... If you keep putting in lower and lower velocities, the suggested twist rate keeps getting faster and faster (wrong!).... The example I tried was a .25 cal that is 3 calibers (0.75") long, and at 1500 fps it suggests a twist of 10.8", but at 1200 it says 9.7", at 900 is comes up with 8.4" and at 600 just 6.8".... If I use the Kolbe Twist Calculator, which DOES provide for lower velocities, for the same LOA and caliber, with a flat base (the Miller Rule does not allow for boattails anyways) and a half-caliber long nose and 50% Meplat (typical for an airgun slug), I get the following for a GS (Gyroscopic Stability, also known as SG) of 1.5 (recommended)....



I use either the Kolbe or JBM Twist Calculators exclusively, as they are the only one I have found based on the work of Robert McCoy while he was working at the Aberdeen Proving Grounds, and are suitable for subsonic use.... There may be others I am not aware of....

http://www.geoffrey-kolbe.com/barrel_twist.htm

https://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

You don't give me the length slugs you are shooting, but it is entirely possible that 17.7" is fast enough, depending on calibre.... For a .25 cal slug that is 0.50" long, 17.7" is actually faster than needed!.... Twist is actually calculated in "calibres", so for a given shape, a smaller slug will need a faster twist.... I have no idea why LW uses the same twist for all their airgun (pellet) barrels.... Current thinking on pellet barrels is that they should be between 100-200 calibres twist rate.... so for a .25 cal from 25-50" twist....

Bob
 
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Below is the comment I added to the Bob Sterne post quoted above.

"Even the Kolbe calculator is inaccurate at speeds between about 1000 and 1200 ft/sec. Bob McCoy's equations were really for rifle bullets fired at supersonic speeds, and appear to be very simplified versions based on the form of the equations used in Intlift. The equations he created are not accurate as speeds approach Mach 1 and were, I believe, never intended for detail use at such speeds.

The other problem is that the bullet is assumed solid with no hollow points or base cavities, all of which will affect the centre of gravity position and thus the twist rate needed for stability. Intlift on the JBM site will calculate some of the parameters needed, but you would still need centre of gravity position and other things like estimated inertias, which you are not likely to be able to obtain. I do not know of any other available software which can give an estimate."

There are many factors which decide the minimum twist rate for slug stability, all of which are driven by the slug and the atmospheric conditions, not the barrel which is only there to provide the slug speed and spin rate ratio through the twist rate. For true gyroscopic stability calculations you need the aerodynamic moments about the CG, the projectile moments of inertia, the air density and projectile diameter, and the twist rate. You do not need the projectile velocity or spin rate, as these are fixed relative to each other by the twist rate. However, the aerodynamic moments will vary with speed, so you will need the speed for calculating the aerodynamics.

The aerodynamics are also affected by the slug shape, boattail slugs need more twist than cylindrical base ones. The slug shape, both external and internal shape, and length will also affect the moments of inertia and the relationship between the axial and longitudinal inertias, which will have a first order effect on the required twist rate.
 
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Triglav said:
Just like springers, some shoot slugs well, others do horrible. The thing is, that two different rifles, of the same model, usually won't shoot slugs the same.
Click to expand...
Unlike pellets slugs do not conform to lands and grooves of a barrel. As roachcreek has posted many times you need to slug sized to the groove diameter. It takes a little more work to shoot slugs accurately.
 
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I know, but then again, no barrel of your typical mid-end springer is the same. High-power PCPs use the so called slug liners that are capable of stabilizing different kinds of slugs, in accordance with their lengths, weights and even materials, as some slugs are copper, or otherwise zinc coated. These liners are usually categorized into several instances, like: Liner A,B,C; Which kind of goes like this, A for weights up to 25 gr, B for up to 30gr, C for up to 45gr... or X for numbers in between. Factors like the groove depth, twist rate, barrel length, even the tune or vibration factors, all play a role in the stabilization of different kinds of projectiles, everything from diabolo-shaped pellets, to hybrid slugs, to regular, hollow point slugs... et cetera. I know, that most springers and entry-level PCPs, just like .22LR rimfire rifles, in .22 caliber, usually have barrels with 1:16" bore twist rates, while some manufacturers like Gamo also like to go and play a dangerous game of "catching the twist rate", which unironically also goes by the name of "let's do our thing, it'll work...", and then it doesn't. Gamo once or twice decided to give one of their break barrels a barrel, with a 1:12" twist, and all of the JSB heavies made targets into something you'd expect from a birdshot blast with a sawn-off double barreled from point blank. Since then, they've just gone with 1:16" and they're much better off with that.
 
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iAMzehTOASTY1 said:
Better off just trying and buying; then using best results personally acquired
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That's what I've noticed, that everybody has shot a ton of slugs and drawn their own conclusions. We're all spending a heck of a lot of $$ to come up with (What I would assume) would be relatively similar results. This said, keeping such meticulous notes that you could draw conclusions would be difficult for even someone who is as anal as I am. As @Ballisticboy mentioned, you would have to note the humidity and atmospheric conditions for every experiment, and would ideally want to have the most similar conditions for the whole test. Even as the cross winds change and swell as shown on this thread showing wind flags in high wind. That shifting wind could destabilize the projectile when it would have been just fine if shot in 5MPH less wind...but maybe you just increased the velocity by 5FPS, which would lead you down a rabbit hole. "Was it the wind, or was it the velocity"

I've noticed that slugs perform opposite what pellets do in that they will often stabilize with an increase in velocity, where pellets will start spiraling when you increase the velocity. This can also make experimenting with slugs more difficult.

It would be a great start if FX would release Matt Dubber's notes when he was experimenting with rifeling and twist rates on slugs...but it still may be missing the atmospheric data.
 
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