Large BC Differences — Due to Barrel Differences? 🤔

[OldSpook]

According to this Chair Gun simulation which matches the parameters specified the difference in TOF at 50 yards between the two projectiles is about (.022/.018)*100 or 22% give or take a bit. At 100 yards the difference in TOF is about 20% also. The equation for acceleration is non-linear so one would expect to be able to see a measurable difference between the two projectiles. If one does not maybe it is an accumulation of smaller factors like, but not limited too, spin drift, engraving of the rifling, and perhaps the change in form factor due to obturation as Arzrover suggests?

I am not looking for an argument here. It looks to me like the differences are going to be pretty hard to pin down when you are chasing nits this small? You are going to need a bench indoors (or a large sample set) and a controlled wind to figure out this one.

 

[tjmac]

I have never calculated the BC for any pellet I have shot over the years. (23 years of FT shooting so far) I shoot as many different pellets through a gun as I can find and choose the one that hits the target most often from 10 to 55 yards. Once I decide on a pellet, I buy 20 000 or so and shoot in every situation I can. I shoot in varied wind conditions, level ground, up hill, down hill and any other iteration I can think of. In short, I learn from shooting what the pellet will do in various situations. Not always right, but I am certainly right more often than I use to be.

Tim

 

[beerthief]

I find the trouble with all this information is I Spend Way To Much Time Reading And Not Shooting .

 

[Ghostranger169]

Some of the info being discussed in this thread isn't practical(or useful) to some shooters, which is fine. I got into shooting pcp airguns in part, to try to replicate the fun of making(and calculating the ballistic solutions for) 500 -1000 yard shots using a powder burner(.308, .338 Lapua, etc..) using an airgun at home in a 250 yard long field by my house. I find it comparable and use similar skills. Because shooting long range PBs is expensive, hard on the shoulder/cheek/ears, and requires a trip to a remote shooting range, I hoped to get more(and cheaper) trigger time using same scope that I do on my powder burners. Now my PBs are collecting dust alongside supply of hoarded ammo, LOL.

In my experience testing, barrel orientation/clocking/indexing definitely has an effect on POI(I index my barrels(when possible) into the highest POI, or 12:00, position(which minimizes this variable as much as I am able to). Perhaps the same 2 barrels indexed differently would exhibit performance differences in certain winds also(i.e.- slightly alter BCs)? I've seen instances of barrels indexed in a particular orientation, exhibiting unpredicted "jumps" in POI when power levels were increased.
Too fast or too slow of a twist rate(even when fired at speed which ammo performs well at with a better suited twist rate) can definitely destabilize a pellet or slug(probably having a negative effect on BC) also.
Obturation could possibly affect aerodynamic projectile performance, as well as, muzzle velocity?
The "non-weather" information required by ballistic calculator apps(Strelok Pro, Chairgun, etc...) that would be used to help predict where a fired projectile will hit down range goes as follows:
Projectile info- *measurements, BC, MV(which is usually expressed as an average of a string of slightly higher or lower velocity shots), and drag function(which is an approximation and not absolute either)
Scope info- *height above the center of the bore, *zero distance, *reticle type and *click units
Gun info- *barrel twist rate
*Everything with a star by it can be physically measured or observed(with the possible addition of MV using a good chronograph). This info SHOULD be the "no brainer" part.
And, as I understand it, those of you with a Lab radar do have the ability to measure BC of projectiles at different ranges(as a function of velocity decay over distance/time.

So, when confronted by actual results which are very different than predicted results, I try to find out why and fix it so that in the future actual results agree with predicted solutions. It's usually something like- I'm shooting a new gun/new tune/new ammo/different speed/etc... I zero the rifle and start shooting farther out like 100-150 yards or so. I shoot a relatively tight group, but that group is .6 of a mil lower than predicted at 100 yards in near calm conditions. The (relatively)tight group part tells me the ammo is relatively stable, so I re check all values entered into Strelok Pro. If they are all correct, I start tweeking the BC lower until all the predictions for the longer distances are very close to actual results.

 

[Arzrover]

I'm with you tjmac and beerthief. I've just noticed over the years that some combos were much better than others and am curious about the "why" My USFT arrived with a 25 shot target done in LD's tunnel that was about nickel sized and I shot it plenty to know that in good conditions , it would do that. It was VERY poor in the wind, though. I had to hold off well more than double my CRX so I went early to one of LD's matches and took 3 barrels with me... 2 HW's and an LW poly I had acquired (thanks Wayne). After quite a bit of testing , the LW got the nod. In that process, the RN10 was compared and was fully equal to the USFT at 51 yards and LD could find nothing at that time that would outshoot either. Also note that the RN10 had the smallest vertical dispersion. The wind was roughly at 90 deg and varied a lot so we zeroed in the tunnel and shot 10 shot groups and measured the magnitude and dispersion of each barrel with no holdoffs.
I get what you're saying , Oldspook, but I don't think 2.5 times the wind sensitivity is exactly a "nit". This is about what we saw best to worst. If you talking about benchrest or FT, a 10% difference in the wind sensitivity would be a possible 20% increase in difficulty to calculate. Again, I'm just looking for the "why". I've fitted a good number of barrels to different models of rifles and so far, the barrel on my USFT is as good as it gets for wind.

Interesting stuff, to me. Looking forward to other input.
Bob

 

[Franklink]

I've not seen even a small percentage of the barrels and guns that Bob has but it is strangely coincidental that both of the best we have seen are polygonal barrels in USFTs.

When I first got my USFT, I remember being impressed with how little it seemed to be affected by the wind. I'd been shooting FT for a bit by then, using a couple different HW springers, and a pretty dang accurate Brocock with a traditional LW barrel. When there was ANY wind, shooting the poly in the USFT felt like cheating, compared to my previous sub20fpe experiences.

The real big experience that got me thinking that I might have something special was at 2 winter matches at Ben Avery shooting facility. The wind was absolutely nasty both months. Huge gusts and baseline winds that would push my body around. Everybody was struggling. I was hearing guys say they were holding multiple mils off, while I was only needing to hold an inch or two into the wind, sometimes not even off the faceplate. Everybody I heard cite their hold offs was needing MUCH, MUCH more hold off than me, and then remarking how the pellet still hit on the far side of the kill zone. One of those matches I was the highest score of all classes by 5 points and the other I was the highest by 3. Those kinds of margins are atypical in field target.

https://www.airgunnersofarizona.org/march-2019-match-report.html

https://www.airgunnersofarizona.org/january-2019-match-report.html

I share that not to boast or brag but to illustrate that some guns are just better in the wind and require less hold off, which is the premise behind why jungleshooter started this discussion. Had I been shooting a different gun those two days I'd have scored down around where everyone else did.

I was shooting the same JSB 10.34grain pellets that many others were shooting that day, at the same general field target legal speeds (in my case, from about 905-930fps with the USFT). I wasn't even shooting sorted pellets, they were washed and lubed, but otherwise straight out of the tin. I wasn't using a scope that allowed me to range better, the opposite those two months actually.

I've since shot a really accurate .20 in field target for a handful of matches. It is capable of the same outstanding accuracy of the USFT, but requires more hold off in the wind. I've measured those .20/13.73s with BCs as high as 0,042, while the .177/10.34s from the USFT haven't ever measured higher than 0.036. So whatever makes the poly barrel shoot so well isn't explained by an improved ballistic coefficient.

The only unquantifiable variable is that barrel (and perhaps Bob's idea of the way the air hits it from that gun). And, there's nothing physically remarkable about the barrel. Pellets pushed through it look like pellets pushed through other poly barrels. Rifling is regular polygonal shaped rifling.

As Bob says, the "why" is intriguing (and frustrating for those that are hoping to replicate the results by following a recipe, ie: x pellet at x speed from x barrel producing a BC of x requiring a holdoff of x in x mph of a x degree wind at x yards).

The real hope (and the reason some of us embrace the minutia and pick the so-called "nits") would be to stumble upon the answer to what makes a barrel seemingly magic. If someone completely figures out the "why".........the implications for high power/long range use........whoa.

 

[JungleShooter]

Thanks a lot for all this very interesting input!!

Wow, this really is one of the deeper rabbit holes in airgunning....


Ghostranger,
▪ like you I very much enjoy the level of ballistic complexity that airgunning offers — but without the expenses or the hassles in distance, licensing, and legality that firearms require.
AG's are awesome!!

▪ And shooting AG at looong ranges makes the whole thing so much more interesting and exciting — the ever enticing challenge to hit targets further and further out, just armed with data and a calcuator: dialing, squeezing, hitting — shooting by the numbers (like instrument flying "by the numbers").



qball,
▪ I can see how the engraving caused by the rifling might account for a small amount in performance.... Good point.
▪ The published BC data is taken by the ballistic calculator and is adapted to the atmospheric conditions I feed it to calculate my current shot, so that should be fine.


Arzrover,
I can see how blowing out the pellet skirt (obturation) can have a more significant affect on BC, as the whole shape of the pellet is changed. Good point. I would expect a higher BC, too.....


Franklink,
the higher BC you measured when shooting downwind is to be expected.... — BC tests require no wind conditions, certainly not head/tail wind as those will severely skew the results.


Yes, some of us like to tinker with our hands, others with our brains, others don't tinker, they concentrate on shooting:
I'm learning that our airguniverse is pretty wide, very different types of airgunners out there. So neat we get to meet here and compare notes.


The experiences that Franklink is reporting certainly seem to point to the uncomfortable conclusion that different barrels make LARGE BC DIFFERENCES.
It really seems like a test would be needed
▪ to shoot the same tin of pellets,
▪ with two rifles that prefer the particular pellet,
▪ both set to the same MV, and
▪ shot simultaneously by two shooters,
▪ while significant wind is present.



Franklink, congrats on both a very awesome gun, and very awesome shooting!

Now, please, bear with me, I'm not trying to attack anyone, just wondering how the mentioned BC values were calculated:
.177cal: 10.34gr: BC=0.036
.20cal: 13.73gr: BC=0.042
They seem, in my opinion, very high, when compared to the BC values that HardAir test results show for the .177 caliber....



Great discussion. Thank you!

The tentative conclusions I draw from this thread so far:
(1) Most barrels will influence the BC of the projectile (pellet or slug) a little. Therefore, published BC numbers (by mnfctr or users) are very often useful.

(2) For extreme conditions (extreme long range, extreme high wind velocities), a BC test with the particular barrel—projectile combination will produce more precise ballistic calculations. Instead of doing a real BC testing, the trajectory can also be "trued" in the calculator.

(3) Some barrels will make a big difference in BC. These barrels, if they produce much higher than usual BCs, require BC testing or trueing. And if I ever get one of those, people will have to pry them from my death-cold fingers.

Matthias

 

[Franklink]

I recognize and appreciate all the work that went into collecting the information for the BC information on Hardair.......and no disrespect to whoever put together the BCs on Hardair, but they're junk.

The measurements listed for some of the pellets were taken at very unrealistic speeds. For example, the .22/25.4gr Monster Redesign shows a speed of 718fps at the muzzle (for a predicted BC of 0.039). Yeah, it probably does have a BC that low when it's going over 200fps less than optimal for that pellet. Another example: the Hades .177/10.3 was measured with a muzzle velocity over 1000fps. WAY too fast, although the BC of 0.029 that they list might be about right.

Also, I can't find where Hardair lists the BCs for the two pellets that I used in the example. I don't see any BCs for .20 pellets, and neither of the JSB 10.3s that are listed are the "Diabolo Exact Heavy" that are so prevalently used in field target.

Generally though, yes, the BC's that I've measured are higher than typical online literature values. I live at 5600 feet above sea level, which accounts for at least some of the difference. My standard BC collections process is the speeds at two distances method, and I like the second distance to be 50 yards or more. I like to average ten shots over the chronograph at each distance for the fps values that get placed into whatever ballistic calculator that I'm using. The newish one from Miles is pretty nice, "EasyBC" https://play.google.com/store/apps/details?id=com.gpc.easybc

Doesn't matter which drag law you use (within reason: G1, GA, GA2), surprisingly, they will all give very similar BCs.

My BC collection method is cumbersome and doesn't give as good of data as a Labradar, but it's what I've got and better than using some random barrel's BC from the internet. When I work backwards from the BCs that I've collected and plug it all into Strelok on guns where I've mapped out the trajectory, they are typically pretty dang close, usually within a 1/10mil click or too. By mapped out the trajectories I mean where I've shot at various distances and compared actual impact points to strelock predicted ones. On my field target guns that is from 10-55 yards and on my two long range guns/configs, 10-140 yards.

(As a side note on all this, before I got into field target and the science of trajectories, my pdog pesting was mostly just kinda walking shots in, hope and a prayer type stuff. Since I've learned about trajectories and BCs and wind drift etc. my pdog pesting has been quite fruitful. I've actually had to really ease up on them in some places, for fear of complete eradication.)

Yes I agree. This has all been great discussion. It's awesome to see the insight and opinions shared. Helps me understand better and think about things differently. (Might need to try some obturation tests. Arzrover and I have talked about how to devise such a test but I haven't yet done it.)

 

[Franklink]

There is also another factor which likely changes the bc... obturation.
In my example above, both rifles had relatively small ports. In the USFT case, it's to help with consistency in an unregulated rifle. In the RN10 case, it is a design tradeoff to allow straight into the barrel loading and was never really intended for 12 ft lb and above. When the air blast is damped by the small port, the swelling or reshaping of the pellet is less than one like a Red Wolf where full bottle pressure is applied briefly to the pellet. I believe this was a large factor in CPH's doing so much better at 20 ft lb than the comparable pellets in the Premier golden era. Very little skirt to obturate and harder alloy.

So to me, a longer, softer pulse to achieve the same velocity SEEMS like it should shoot better in the same barrel by changing the shape of the pellet less. Slugs would have little change, so they would show less preference .

Bob

From your comments on the hard alloy of the Premiers back when they were the cream of the crop......are you thinking that bulged out skirts (obturation) is a bad thing? ie. JSB designed the shape of the pellet to be best at that shape, and when it changes it is no longer as aerodynamically stable?

See, that's where this type of discussion is great (changes our preconceived notions), b/c I've always wondered if over-obturation could be a good thing on really hollow skirted pellets, like blowing that skirt out so wide that the pellet is less of a diabolo shape and more akin to (dare I say) a slug.

 

[Arzrover]

Interesting idea but how would you design for it given the enormous differences in the rifles it would be used in?
A more serious concern for me would be uneven obturation, causing imbalance or different air flow. The 25.4s have little skirt to obturate so maybe that's why we can drive them so fast before they get wonky.
A side note... I was given some Turkish copies of some JSB's to test and some weren't too bad but the 18.1 copies were truly horrible. On examination, I really couldn't see a reason, either. But then, I have a couple of batches of JSB 18.1s that are equally bad. It would really be interesting to know why...
Bob

 

[dizzums]

choked or not choked .. projectile shape is a huge factor in bc .. so a pellet goes in with bc of X and comes out after swaging in a choke with bc of Y .. i really have no idea about bc, i just see how the gun shoots .. if its off alittle from the software projection could be bc OR shooting angle .. shooting 'down' at a target sighting in at 20 'does' matter ..its not a flat trajectory if thats what in the software setup .. figuring bc means diddly squat if you missed that ..

 

[Michigander]

In my experience, stability has a major effect on velocity retention, and hence measured ballistic coefficient. Get into a situation where stability is marginal and you will find that the pellets that show signs of keyholing also have lower retained velocity. Happened with my Streamline and JSB 18.1 pellets. Also with a Daystate and .20 CPs.

Do we really expect all pellets shot from all guns to be perfectly stable? I think reduction in BC due to minor instability happens to some degree all the time, which is why published BC values have to be taken as approximations only.

 

[Franklink]

Interesting idea but how would you design for it given the enormous differences in the rifles it would be used in?
A more serious concern for me would be uneven obturation, causing imbalance or different air flow. The 25.4s have little skirt to obturate so maybe that's why we can drive them so fast before they get wonky.
....
Bob

I'd hazard a guess that my idea of how to examine the obturation from different guns for comparison is much more simple than what you're envisioning. I was thinking of shooting pellets into something like blanket batting or pillow stuffing, something light and fluffy but yet fibrous enough to slow the pellet down without further deformation from the impact/interaction with the stopping media. I've got those sections of steel square structural tubing that should be more than long enough. Leave the ends open but still put a rubber mulch pellet trap behind the outgoing end, just in case. Use a broomstick as a ramrod of sorts to push out the stopping media (hopefully containing the now-fired pellet) to allow for examination. Crude, but potentially helpful to "see" if there's a clue in the obturation.

As for uneven obturation, for those that don't know, the USFT has the unique swing breach transfer port that results in air hitting the pellet squarely in the skirt, versus other designs that have a potentially turbulence inducing 90degree turn for the blast of air located much closer to the rear of the waiting pellet. USFT has a 90 degree turn too, but it's further away from the rear of the pellet than some more traditional designs. Maybe the air in that longer straight shaft of transfer port in the USFT has a chance to become more laminar before getting to the pellet so causes more even obturation? (More "even" being in a 360 degree manner, versus maybe blowing out one side?)

The .177/10.34s also have very little skirt to obturate (like the .22/25.4s you mention) so I dunno.

 

[Franklink]

published BC values have to be taken as approximations only.

Yep, I agree 100%

 

[oldsparky]

Can I ask a question? When measuring actual BC in the field. Is it common practice to be sure each barrel is at the same degree and height?

thanks,

 

[Arzrover]

All mine are done with a LabRadar and from the same bench and typically at the same 75 yard target.
I would think that the angle would have to be significant to get noticeable differences.
Bob

 

[Franklink]

Yeah, similar here too. Near speeds from the same little shooting table and far distance speeds taken from chrono on the ground. And my creek bottom land is flat as can be. I'm not moving the chrono around within the 10 shots that I used for the average speeds either, so all 10 shots (that make up the average speeds at each distance) are taken with the pellet path going over the chrono eyes at the same angle.

Interesting thought though (ie. would having the far chrono drastically higher or lower than the gun's elevation help or hinder a pellet in maintaining or losing speed. I don't think a steep angle would matter much for the muzzle speeds, but it might for downrange speed measurements)

 

[Franklink]

Just remembered that we shot my high power gun over Arzrover's Labradar and the BC we got from that was effectively the same as the BC's I've collected using the chronograph at two distances method.

With his Labradar we got a BC of 0.049 with .22/MRDs and in normal conditions (without the tailwind I cited earlier) I have measured them at 0.048-0.052 using my method numerous times.

 

[oldsparky]

Very interesting conversations. I have never considered BC I have only found a bullet that was accurate. I am talking firearms. So I am all ears here. No recommendations just questions. Something that is so different is a a given load has a consistent energy when starting. And air guns can get power from a combination of energy and barrel length not necessarily the same starting pressure. Wow!

 

[Arzrover]

Well... I had hoped to put some time in on very specific tests before posting again but things have a way of interfering.
However... a new project has caused more questions ...
I built a special choked poly 177 to be cf wrapped for Franklink's Vet Standard . After some teething issues we were able to get it to him after the Utah match this last weekend . When he was able to install and test it... it was very good or at least adequately good in no wind conditions but horrible in windy conditions. At this point you're probably thinking "but it's a 177.. what do you expect ?". In that vein of thought, he breaks out the trusty USFT and what do you know, it shoots extremely well and a fraction of the wind deflection . Now the poly on the Vet was 1 in 17 and the USFT was 1 in 36 so MAYBE this has something to do with it but as I have 2 extremely good samples that are wonderful in the wind and are 1 in 17, I'm not convinced of the importance of the twist rate here.

Another thought as I was reading back through this thread... on the experiments at LD's place, all were tested on a USFT, so differences in obturation and pressure profiles would not have been a factor.

Soo... it's kind of bumped the research priority again. Hope to post some results this coming week.

Bob