Will the twist rate of a barrel change the speed of a particular round? Lets say I have a 22-250 load using a 55grn VMax and 34grns of IMR4895... If I shoot it out of two rifles with 26" barrels. One is a 12 twist and the other a 14 twist. Will the speed of the round change at all between the 12 and 14 twist barrels?

Thanks

Mudruck

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"Our Country won't go on forever, if we stay soft as we are now. There won't be any America - because some foreign soldiery will invade us and take our women and breed a hardier race!" - CHESTY PULLER, USMC
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Utah Varminters Association http://www.armscenter.com/uva

Yes, you should certainly see a velocity decrease with the faster twist. I don't really know how much difference, but I would guess it will be about 100fps less. Any real world chronograph work done out there?

The wildcard in this is that you could see that much difference in two different barrels (with different chambers) and not even change the twist rate so I'm not sure that a good scientific test could be performed.

Thanks for the info.

Mainly I was looking to see what velocity I should be expecting from some of the published V-Max loads from Hornady. They list a 14 twist and Im using a 12. If its not a major change then Ill just go with the data I have and go from there.

Thanks

Mudruck

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"Our Country won't go on forever, if we stay soft as we are now. There won't be any America - because some foreign soldiery will invade us and take our women and breed a hardier race!" - CHESTY PULLER, USMC
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Utah Varminters Association http://www.armscenter.com/uva

I don't doubt that there would be a difference, but I doubt you'll be able to prove it's the twist rate. Too many other variables, to small a difference. I also doubt it would be 100 fps for a small difference like 12 to 14.

Ditto Cannuck - There is not a considerable difference. There is some data which indicates that gain twist barrels do show some increases in velocity, however, gain twist barrels are very expensive to cut.

The function of twist is ONLY to stabilize a bullet of given length/weight. In other words, it makes no sense to consider a slower twist in order to gain velocity. What is the use of velocity if your bullet is tumbling towards the target? Folks have also played around with the number of lands in a barrel to improve velocity. However, there is so little difference in velocity between a three land barrel and a four land barrel.

At the end of the day, powder is what makes bullets go. More powder, more velocity - however, the laws of diminishing returns applies all the way until you reach the Kaboom (Ka) limits of your chamber.

Kind of an aside, I guess, but I believe that gain twist barrels will increase velocity. At least that is what happened with mine. Of course it was only a one caliber test, but it sure gained fps, and was a lot more accurate too boot. FWIW - Dan

Lots of variables to think about here.

Let's, for the sake of discussion, assume that the barrels are identical except for twist rate.

There's every possibility that the faster twist rate will create MORE back pressure because it causes more resistence to the bullet's forward motion.

More back pressure or resistence to pushing the bullet down the barrel will increase internal ballistic pressure. (Powder generates pressure by being confined under pressure while ignited.) In theory, more pressure results in more veloctity. But that may NOT be the case for some other complex ballistic reasons.

It's doubtful that the difference between 1:12 and 1:14 will be significant. But the difference between 1:9 and 1:18 may be more marked.

Now . . . the next variable would be the characteristic of the powder to increase pressure internally under varied resistence. Some will increase pressure with resistence. Some may not.

Lots of "voodoo chemistry" going on with powders. Buffers, retardants, stabilizers . . . It never fails to amaze me that one will produce higher velocties at lower pressures than another -- or that this relationship between velocity and pressure will shift with different weight bullets.

-- I gotta go chant over some brass . . .

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.223 Ackley Improved Wildcat Forum:
http://www.hotboards.com/plus/plus.mirage?who=223ackleyimproved

[This message has been edited by Genghis (edited 04-04-2002).]

Genghis-don't forget to sacrifice a chicken. LOL - Dan

Somchem Ballistic Proof Lab did extensive testing with speed and twist rates and the conclusion was that the effect of changes in twist rate was statistically insignificant.

I have seen one gain twist barrel that seemed faster than normal and I have also seen a gain twist barrel in 6mm that was not.

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Gerard Schultz
GS Custom Bullets

I calculated the spin effect on pressure for a 30 cal once a couple years ago. 22 cal should be even less than the piss ant amount for 30 cal:

BULLET SPIN EFFECT ON PRESSURE
The barrel twist rate is 1 in 10"
If the bullet is V= 1100 fps the the spin = [12"/10"][1100] revolutions
per second =1320 rps
If the bullet weighs 147 gr = .021 lb
The bullet has a diameter of .355" =.1775' r =.0148'
The moment of inertia of a cylinder is [M][r]squared/2
Mass = wt./g = .021 lb/32.2 fps squared = .0006522 lb sec squared/ foot
Moment I = [.0006522 lb sec sec / ft][.0148'][.0148']/2 = 7.13 EE-8 lb
sec sec/ ft
E = [1/2] I w squared
w = rotational velocity in radian /sec = 2[pi] [rev]ps = radians/sec =2
Pi 1320 =8294 radian /sec
E[rotational]= [1/2][7.13EE-8][8294][8294]= 2.45 foot pounds
Force rotation = E/d = 2.45 ft lb / .2933' = 8.35 lb
Pressure rotation = F/A =2.45/.099 sq. in = 24.75 psi or a .2% effect