there is simple formula:
twist rate (inch) = 150 * bullet diameter (inch) squared / pellet length (inch)
but how the pellets weight play in the role ?
The total wt of pellet doesn't really do much except the fact that when all else being equal, heavier bullet are longer.
The wt distribution also has to do with the stabilization, if the center of mass of moved to the back, the heavy part will tend to move to the front (so it tumbles). With the same length, a splitzer boat tail will need faster twist than a round nose. On boarderline cases (like the 215gr .303 of the old days), the tail of the bullet actually moves in a circular way and may take up to 200-300m to totally stabilize.
Over spinning, while not as serious as underspinning, will cause the projectile to point at a slight angle over flight (not that most people will notice).
The question is, pellets is nose heavy and has a skirt for drag hence self stabilize without rifling, does the twist rate really matter, should we use the slowest prossible twist to prevent over spinning?
BTW guns with really fast twist rates are also a mission to handle, you can feel that when shooting a lighter center fire from left hand side. But this is not the major problem, the real problem we have is the suppressor coming undone. With 99+% of all barrels (unless you have a colt 1911), the twisting action will unscrew the barrel attachment. That's why on a 3 piece suppressor, you always get some parts coming off easier than others. The use of 1/2 UNF for 22LR and airgun is actually wrong, it should have been a mirror image of it instead. Many suppressed guns are made with a reverse tread or use locking lugs instead. I know it makes a difference with .22LR but not sure about airguns....
Oh good a nice thread again!
Twist rate is quite a fascinating subject, one which I have studied as much as I could find, most of the articles/ papers I have read have been Black powder related but they transfer well to air rifles.
Firstly, the larger the calibre the slower the twist can be, so with a .177 barrel the "std twist" is 1 in 12, a .20 cal barrel would be fine with a 1 in 14 twist, that is for "standard" wieght pellets.
The reason for this is that twist rate is directly related to velocity of the pellet/ bullet in the barrel (internal ballistics), i.e. the faster a pellet/ bullet travells the faster it spins out of the same twist barrel.
So if you want to swage/ cast heavy for calibre pellets for your .177 or .20 etc, rifle you have two basic ways of making sure the pellet will stabilize; 1. shoot it faster or 2. tighten the twist.
Basic maths heavier pellets most often shoot slower so twist rate is what we can manipulate. BUT here's the kicker (although I am sure there is some lenghty equation to prove it in theory), to arrive at the best twist rate you have to experiment and fine tune. So you might want your 30gn pellet to do 900fps in a 1 in 10 twist but the best u get is 820, you have to tighten the twist some more.
what gonna happen if the twist rate too fast and caused over spinning? does the twist rate really effected to killing power(damage)?
here is other guy had to say:
There are 2 desireable effects controlled by twist rates and they lie at separate ends of the spectrum. On one end of the spectrum the high twist rate procuces a high gyroscopic stability of the projectile in flight (this is desired by serious target shooter or long range shooters. On the other end of the spectrum the low twist rate produces minimal flight stability but upon impact tumbles more readily and delivers more of the projectile's energy into the target and thus causing maximum damage(this is desired by hunters or mid-range snipers). The more specific the use of the gun is by it's owner the closer to one or the other end of the spectrum it's twist rate. In the middle is balance and for most general purpose.
Just for reference, during the Vietnam war Armalite (manufacturer of M-16 rifles) decided to push the effective range of the M-16 out past 500 meters by changing the barrel's twist rate to 1:7". The new twist rate stabilized the flight of the .223 round and pushed the effective range of the M-16 out to 680 meters. However, after 4 months the US Army reported back to Armalite that their soldiers had to shoot the enemy with multiple shots to brimg them down. The new twist rate was giving the .223 rounds too much gyroscopic stability to push cleanly thru fleash and leaving less leathal wound canals. Armalite responded by going back to the original 1:12" twist rate. They found that it was the best balance between maximum effective range and delivering maximum wound leathality.
Can anyone tell me why do we still need rifling if pellets will not tumble even if shot from a smooth bore?
I really don't think twist rate matters to pellet as much as bullets. Militry stuff rely heavily on the tumbling effect of the bullets as they are not allow to use soft and hollow point (but 223 FMJ is just as nasty). Big game hunters don't want a tumbling bullet as it must penetrate very stiff tissues and bones. So it really depends on what you do. With pellets I don't think it tumbles on impact anyway so the most accurate pellet wins although you might not get pellet expansion with heavy pellets/slow velcoity......
So Boondock, sound like you need to buy a set of swage dies and press to make Pellets of different lengths to play with........
BTW the US army did go back to 1:7 barrels in their M16A2 but with a longer bullets, it doesn't tumble as readily as the 55gr 1:12 but penetrates and shoots better........trades offs and trade offs..........
I finally measured the twist rate in the BAM .177 cal. last night.....
It is 1 turn in 18 inch and has a counter-clockwise twist in the direction of the pellet....
Now you can start calculating......, for 800 ft/sec (typically 12 ft.pound) the pellet will rotate at 32 000 revs per minute and at 1100 ft/sec (typically 23 ft.pound) the revs will be 44 000....
No wonder the feral pigeons don't like it...., they get knocked by the rev's......!!!!!
Use that information in your formula to determine the shortest pellet that will still be stable, depending on the Energy level you are choosing....
Will come and measure your twist rate on the Walther .20 tomorrow to do the same calc's.....
At 800ft/sec pellet leaving the barrell, the spin rate is 1 revolution per 18 inches......, and 18 inches into 800 times 12 is 533.33 times..., per second.....!!! Times the 60 seconds per minute, is ~32 000.......
It has nothing to do with the length of the barrell.....!!!!
Malan :eek :! :thinkerg
i didn't mean 18inch barrel length.. but 18inch rifling twist rate..
say 32000revs/min ,
800ft/sec = 244m/sec,
pellet will take at least 0.12sec to reach 30m target,
if 533revs/sec, how many revs spin for 0.12sec?
still less than 66revs spin to reach 30m target.
above theory only based on ideal condition..