rifling and twist rate

[Muppit]

 i have been on this forum for a while now and have read so many posts asking about the best barrel length and the longer the barrel the better fps you will get, and im sure that could start an arguement among many. i have not once rean anything about the twist rate of the rifling. now mind you this is very important in powder burners, especially muzzle loaders. differant twist rates affect differant projectiles in differant ways. so whats the deal in air guns? has any one given this any thought is this why lw barrels are better? ........dave

[rsterne]

Most air rifle barrels are quite similar on twist rate, running between 16-18" from what I understand, but most manufacturers do not publish the twist, nor, as far as I am aware, is there a standard.... LW barrels are 1 turn in 450mm (17.7") in all the popular calibers, I know because I asked them.... There is a big difference in the required twist rates below the speed of sound and above, there being a big "discontinuity" in the stability curve right at Mach 1.... Here is a graph showing why bullets that are stable above and below the speed of sound can be unstable at Mach 1....



I don't remember the bullet dimensions I used to generate that chart from the Border Barrels website, the purpose was to demonstrate how tumbling can occur.... A "normal" stability factor is about 1.5-2.0, with under 1.0 being unstable, and over 3 or 4 being overstabilized.... The military use around 1.5 to 2.5, Benchrest competition shooter often try to get the stability down to about 1.3....

In general terms, longer bullets/pellets in a given caliber require a faster twist.... Smaller calibers require a faster twist (in inches) as the twist rate is actually relative to the diameter (it is initially calculated in calibers).... At subsonic velocities, if the twist is sufficient for the bullet/pellet to be stable at Mach 1, then it will have additional stability as the velocity decreases (the opposite of supersonic flight)....

Bob

[robert w]

1 thing it takes more power for a fast twist like 1 turn in 12" vs 1 turn in 17" i keep wonderin how a magnum would do wit say a 1 turn in 15" twist . i have a piece of barrel stock new that is a turn in 14 " and keep thinkin when im not doin anything , turnin it down to fit in 1 of my magnums and seein hoow it works . but i seem to keep puttin the job on the back burner

[rsterne]

If your barrel stock is for a .22 rimfire or centerfire PB then it is too large for pellets.... A .22 cal airgun barrel is only 0.217" groove diameter.... A twist faster than 16 or 18" would only be necessary on very long pellets (ie bullets).... Once you go supersonic, less twist is required.... Overstabilization causes problems too (vertical stringing due to increased magnus effect in crosswinds, emphasizing pellet imbalance, etc.)

Bob

[Muppit]

Bob thanks for the imput. it just seems to me that all this barrel lenght talk could run hand and hand with twist. if you want to try tuning in on barrel length then the twist rate should be as much of a factor. i never gave much thought to what pellet my gun liked until you mentioned it in your first post that longer pellets required a faster twist. my guns all shoot better with a heavery pellet which just happend to be longer. obvioslly that the twist rate may not be the reason but it very well could be. its to bad that companies producing barrels for the 22xx/13xx guns don't offer them in differant twist rates.......dave

[jimmie lee]

Benjamin Discovery .22 is 16:1 I believe.

[rsterne]

I think you mean 1 turn in 16".... 16:1 would be .217" x 16 = 3.47" per turn....

Bob

[rsterne]

I did some twist rate calculations with various pellets in .22 cal.... for a Stability Factor of 1.5 at 1100 fps.... The stability will increase at lower velocities....

Pellet - Length - Twist @ SF=1.5
JSB Exact RS 13.4 gr. - 0.262" - 23"
JSB Exact Heavy 18.1 gr. -  0.305" - 20"
H&N Baracuda 21.1 gr. - 0.335" - 19"
EunJin Dome 28.4 gr. - 0.391" - 16"
Predator 16.0 gr. - 0.404" - 15.5"
EunJin Point 32.1 gr. - 0.480" - 13.5"

So the first three pellets are stable any barrel with an 19" or faster twist.... If we check the longer three pellets in an LW barrel with a 17.7" twist, we get the following stability factors:

EunJin Dome - 1.2 @ 1100 fps - 1.5 @ 1000 fps)
Predator Polymag - 1.2 @ 1100 fps - 1.5 @ 1000 fps
EunJin Point - 0.9 (unstable) @ 1100 fps - 1.0 (stable) @ 1030 fps - 1.2 @ 800 fps

Therefore, the only pellet which would not be stable at all subsonic velocities in an LW barrel (17.7" twist) is the EunJin Point.... It would, however, be stable at 1000 fps and below.... Now, lets look to see if any of the shorter ones are "overstabilized".... Let's use the LW barrel again.... here are the stability factors....

JSB Exact RS - 4.0 @ ~820 fps - 3.0 @ ~1040 fps
JSB Exact Heavy - 3.4 @ ~500 fps - 3.0 @ ~780 fps
H&N Baracuda - 2.8 @ ~500 fps

What does that tell us?.... Well, in a 17.7" twist LW barrel, I would consider the shortest pellets (eg. like a JSB Exact RS) may be overstabilized at under ~800 fps, and this may show up at longer ranges, particularly if the pellet isn't perfect (eg. the CG is not in the exact center of the pellet).... I wouldn't expect this to be a problem with a pellet like the JSB Exact Heavy, as quality bullets seldom show problems with overstabilization at Stability factors under about 3.5.... Longer pellets, such as the H&N Baracuda are unlikely to be overstabilized at any normal airgun velocity....

You can see why Lothar Walther picked 17.7" for the twist they use.... In .22 cal, at least, it works great for nearly all available pellets....

Bob

[Muppit]

Bob can you figure out the numbers for 2400kt .177 cal shooting 645 fps with 10.34 jsb,with 18 inch crosman barrel. thats just a ballpark you don't have to use my numbers but that is what its shooting. this is just for fun anyway unless whatever the numbers say are way off. i,m leaning to a 22 barrel anyway for more knock down power. its just good to have the info to refer to at a later date......dave

[Muppit]

just checked the twist on my titan and thats got a twist of just under 1.25 in 18 inches of barrel

[rsterne]

Do you mean an 18" twist?.... I can measure a pellet here.... The barrel length has nothing to do with the rifling twist rate....

Bob

[.22 Varmint]

   All of this rifling twist stuff it very interesting! Bob, If remember correctly, you said at one point that FX smooth-twist barrels had a much slower twist than average. Would that not result in under-stabilization? How do FX guns manage great accuracy? I am new to this sort of thing, very cool info.

Andrew

[rsterne]

I read that the FX twist rate is 1 in 13 FEET (156").... If so, they are relying almost completely on the "shuttlecock" stability of a diablo style pellet.... Without accurate information (one of the GTA members was emailing FX to ask what the twist rate was).... I can't even speculate....

Bob

[Muppit]

Do you mean an 18" twist?.... I can measure a pellet here.... The barrel length has nothing to do with the rifling twist rate....

Bob

1.25 in 18 sound better  and i ASSUME  that may be standard for crosman. i had a old barrel for the titan and i'm not going to mess up the good barrel to check them LOL.......dave

[jimmie lee]

I think you mean 1 turn in 16".... 16:1 would be .217" x 16 = 3.47" per turn....

Bob

Yup, I think that's what I meant unless I was thinking about an oil to gas mix ratio, LAM (laughing at myself).

[rsterne]

1.25 turns in 18" is a twist rate of 1 in 14.4".... That is faster than I have heard of in an airgun before, however I will use that number.... The 10.34 gr. JSB is 0.250" long.... Using the Border Barrels Twist Calculator, I get a Stability Factor of 1.9 at 1100 fps, 2.5 at 1000, about 3.25 at 650 fps, and 3.4 at 500.... All those values are perfectly acceptable, although approaching overstabilized below 500 fps....

With an 18" twist, the SF would be 1.2 at 1100, 1.57 at 1000, 2.0 at 700, and 2.2 at 500 fps....

Bob

[Muppit]

that seems right  and explains why the 1o.34 work well. i did use a rather rudamentary method to find the twist rate. i took a brass cleaning brush and a cleaning rode to my old barrel not a good one put a piece of tape around it and slowly pushed it through the bore watching the tape spin till the brush came out the end of the barrel. just a good way to check twist rate. i don't think it would hurt to much to do a barrel once or twice  but i would not do it repeatedly.

    so in the end those searching for that last little bit of accuracy  out of there gun and looking at cutting down or tuning their barrel so to speak also need to look at their barrels twist rate and the pellets they are using.  wow someone could get carried away with trying to get the most out of a barrel.

bob is the border barrel twist calculator avalible online? it would be interesting to look at.

[rsterne]

http://www.border-barrels.com/barrel_twist.htm

Bob

[robert w]

If your barrel stock is for a .22 rimfire or centerfire PB then it is too large for pellets.... A .22 cal airgun barrel is only 0.217" groove diameter.... A twist faster than 16 or 18" would only be necessary on very long pellets (ie bullets).... Once you go supersonic, less twist is required.... Overstabilization causes problems too (vertical stringing due to increased magnus effect in crosswinds, emphasizing pellet imbalance, etc.)

Bobrsteren  its a small bore barrel . early 22 hornets , 22 magnum and several others are .211 . i wanted small bore for an ackley improved wildcat which is like greased lightning . but i ordered a 24" barrel they sent me 47" of barrel . same cost as  i ask how much for 24" . must have had a lazy person that didnt want to cut it off . but its just a thought as to tryin it in an airgun. might some day but not now

[QVTom]

I read that the FX twist rate is 1 in 13 FEET (156").... If so, they are relying almost completely on the "shuttlecock" stability of a diablo style pellet.... Without accurate information (one of the GTA members was emailing FX to ask what the twist rate was).... I can't even speculate....

Bob

Bob,  No response from FX.  Either they don't care to share or just don't care to answer.

All this twist rate talk makes for great conversation.  I think due to the rapid evolution of the high powered PCPs, the guns have sprinted ahead of the the barrels.  Much like the PC processors that are released like rats and are way ahead of the software's ability to take advantage of the performance.

Consider an early springer or pneumatic.  They shot relatively light ammo in the 500-600 fps range, the pellets shuttle cock design was necessary to aid in stability; the pellet design may  have a larger stabilization factor than the riffling.  Twist rate was generally 1/16".  Now PCPs are shooting 25-40gn ammo at 950fps and still use the 1/16 twist.  As energy levels approach the 22LR we are still using the same twist and pellet design.  With the transition to conicals in the higher power levels, a slightly faster twist may have an accuracy advantage by adequately stabilizing the projectile with out the  massive nose heavy diabolo design that tend to corkscrew when pushed too hard.

Tom