twist and barrel

[Pthfndr]

For the people who think Benchrest is not real shooting, below is why Benchresters are key in the development of cartridges and firearms. They may look like they aren't doing much, but they come up with an idea, make all the calculations, and then test it out to see if it works. They are the R&D department of the shooting world. If you ever want to suffer brain overload, go to a BR match and talk with these guys.

Toby when Milotrain posted earlier he was talking about ahnother8 and not you.

To the OP
Don Millers easy to use formula coutesy of Precision Shooting Magazine

S = (30*m)/(t^2*d^3*L*(1+L^2))

S is the gyroscopic stability factor. This is the measure of static stability and 1.4 is the minimal (optimal) value.

m is the bullet weight in grains

t is the twist rate in calibers per turn (for example, a 1:13" twist .308 barrel has t = 13/.308 = 42.2 calibers per turn.

d is the bullet diameter

L is the bullet length in calibers.

This equation is very accurate for a wide range of bullets, velocities and twists. There are corrections for non-standard atmosphere and velocity corrections (the equation is built around MV = 2800 fps). The effect of different muzzle velocity is not as important to stability as most people think.

If you want to shoot a 240 Sierra MatchKing out of a 300 Whisper at 800 fps you would use a 8 twist barrel likewise the same bullet in a 300 Weatherby magnum at top velocity would still use a 8 twist barrel.

Barrel length has no significant affect on twist rate.If you look at the formula barrel length doesn't matter and isn't even in the formula.Pistols shoot and they shoot very accurately and most don't have 6 inch barrels on them.

As you increase the twist rate for a given velocity the bullet spins faster.If your bullet has any imperfections or imbalances they are amplified exponentially as RPM is increased and gilt edge accuracy suffers.This only means something to competition shooters and explains the whole over-stabilization theory.

For Joe Q Public a 308 in 10 twist will work for most anything.
For a competition shooter they already have all of this figured out.

[JMP]

Don Millers easy to use formula coutesy of Precision Shooting Magazine

S = (30*m)/(t^2*d^3*L*(1+L^2))

S is the gyroscopic stability factor. This is the measure of static stability and 1.4 is the minimal (optimal) value.

m is the bullet weight in grains

t is the twist rate in calibers per turn (for example, a 1:13" twist .308 barrel has t = 13/.308 = 42.2 calibers per turn.

d is the bullet diameter

L is the bullet length in calibers.

This equation is very accurate for a wide range of bullets, velocities and twists. There are corrections for non-standard atmosphere and velocity corrections (the equation is built around MV = 2800 fps). The effect of different muzzle velocity is not as important to stability as most people think.

Yes, the Miller twist rule is a good approximation. The constant of 30 is used for standard conditions and can be optimized using calculus. Note that you can rearrange the equation to solve for the optimal twist rate.

An even more simple rule of thumb calculation is Greenhill's formula, which is more crude.

Generally, ballistic calculators will rely on velocity, weight, length, and ballistic coefficient. These calculations won't be perfect "in real life". As it is, the BC, which represents a bullet's coefficient of friction is represented by a scalar value, so it's actually simplified since the frictional forces are multidimensional.

While there is no substitute for real life experience, understanding ballistics will get you to where you need a whole lot faster than a trial an error method, which most people use. You can learn to be a good shooter by experience alone, without a full understanding of what's going on, but you'll burn out more barrels getting to where you want to be.

It would appear, based on this thread, that many people didn't make it very far in their study of physics.

I think the common sentiment that faster twist rates are better is that you'll have better results erring on the side of over-twisting than erring on the side of under-twisting.

[phish]

As it is, the BC, which represents a bullet's coefficient of friction is represented by a scalar value, so it's actually simplified since the frictional forces are multidimensional...

It would appear, based on this thread, that many people didn't make it very far in their study of physics.

.

[ar15barrels]

We talking scenar? Wow 3000fps is awesome.

Ever heard of anyone doing that with Varget out of a 26"?

I used to run 46.5gr of varget under 2156's and get 2950 from a 24" barrel.
I was not even getting pressure signs.
On HOT days, I would just break 3000 with that load.

[subscriber]

Tuning long barrels for accuracy - study of vibration and mass damping: http://www.varmintal.net/aeste.htm

[HK Dave]

I used to run 46.5gr of varget under 2156's and get 2950 from a 24" barrel.
I was not even getting pressure signs.
On HOT days, I would just break 3000 with that load.

How did the 2156 behave? Better or worse than the scenar?

[LynnJr]

Tuning long barrels for accuracy - study of vibration and mass damping: http://www.varmintal.net/aeste.htm

Subscriber
Varmint Al(Al Harral) ran a finite element analysis of my 6 Dasher with a tuner on it that set 3 world records at 1000 yards.In his analysis he concluded the barrel was on a downward slope when the bullets exited the barrel and therefore best accuracy was not achieved.
I gave that barrel away at a little over 1000 rounds and the new owner put it on a XP100 pistol.He said it is still shooting very well.Tuners work very well even on centerfire rifles.
Lynn

[subscriber]

Subscriber
Varmint Al(Al Harral) ran a finite element analysis of my 6 Dasher with a tuner on it that set 3 world records at 1000 yards.In his analysis he concluded the barrel was on a downward slope when the bullets exited the barrel and therefore best accuracy was not achieved.
I gave that barrel away at a little over 1000 rounds and the new owner put it on a XP100 pistol.He said it is still shooting very well.Tuners work very well even on centerfire rifles.
Lynn

Based on his articles and your report, Al definitely knows his stuff!

[ar15barrels]

How did the 2156 behave? Better or worse than the scenar?

I doubt you have my old barrel so you would have to test your own barrel...