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How should one go about breaking in a fast twist barrel?

Long time reader, short time poster here. Just thought I'd chime in with my $0.02. I think the reason people get stuck on pressure so much is because it just makes sense. People figure:

faster twist = harder to push a bullet = higher pressure

It also makes sense that pressure will be higher if you think about it from the perspective of energy. The energy of a bullet is equal to it kinetic energy plus it rotational energy. So a bullet fired from a 1:8 twist 223 at the same velocity of as a bullet that comes out of a 1:12 twist 223 will have more total energy because it is spinning faster. How does it get more energy? The only way is to increase the work done on the bullet. Work = force x distance, and the force on the bullet comes from pressure. Obviously pressure isn't constant as a bullet moves down the barrel, but in general you have to increase the average pressure to increase velocity.

With all these thoughts swirling in my head I thought I'd run some numbers. A 55 grain bullet at 3300 fps will have about 1330 ft-lb of kinetic energy. At that velocity a 1:12 twist bullet will spin at 3300 rev/s and a 1:8 twist bullet will spin at 4950 rev/s. Rotational energy is 1/2 x moment of inertia x angular velocity^2. For simplicity I calculated a moment of inertia for a cylindrical shape. After plugging everything and making a few units conversions I got the following:

1:8 twist rotational energy = 5.1 ft-lb
1:12 twist rotational energy = 2.3 ft-lb

Pretty much negligible compared to 1330 ft-lb. If your gun has 22 in barrel, the extra force on the bullet to get that extra 2.8 ft-lb of energy is 1.6 lb. It takes 124 psi on the back of a 0.224-in diameter bullet to get that force. Again, it's important to note I'm talking about the average increase over the length of the barrel and not an increase in peak pressure. Also, due to inefficiency, I would guess the actual increase in pressure to be somewhat higher. Still, even if the increase is on the order of 500 psi the change is less than 1% of 60,000 psi. Somebody with more experience than me can chime in, but I would have to wonder if a strain gauge on a pressure barrel would even catch that.

Long story short. The idea of fast twist = higher pressure makes sense intuitively, but the numbers don't add up.

PS Feel free to double check math, this was a rush job.

Firth,

If you continue to provide accurate data and analysis, and good common sense, you will probably be in trouble here.

You will be messing with the minds of the totally convinced who have no basis for their convictions, but feel very strongly about them.

I'm waiting for someone to chime in and say that your data would be different if a rifle has a left hand twist instead of a right hand twist.

Steve

And what about the Coriolis effect???

Can you get those in the leopard skin pattern...?

DF

You're getting dangerously close to the cornholius effect.

I am guessing then that a gain twist would hep both problems; pressure and stability. Just guessing mind you. Cheers NC

Long time reader, short time poster here. Just thought I'd chime in with my $0.02. I think the reason people get stuck on pressure so much is because it just makes sense. People figure:

faster twist = harder to push a bullet = higher pressure

It also makes sense that pressure will be higher if you think about it from the perspective of energy. The energy of a bullet is equal to it kinetic energy plus it rotational energy. So a bullet fired from a 1:8 twist 223 at the same velocity of as a bullet that comes out of a 1:12 twist 223 will have more total energy because it is spinning faster. How does it get more energy? The only way is to increase the work done on the bullet. Work = force x distance, and the force on the bullet comes from pressure. Obviously pressure isn't constant as a bullet moves down the barrel, but in general you have to increase the average pressure to increase velocity.

With all these thoughts swirling in my head I thought I'd run some numbers. A 55 grain bullet at 3300 fps will have about 1330 ft-lb of kinetic energy. At that velocity a 1:12 twist bullet will spin at 3300 rev/s and a 1:8 twist bullet will spin at 4950 rev/s. Rotational energy is 1/2 x moment of inertia x angular velocity^2. For simplicity I calculated a moment of inertia for a cylindrical shape. After plugging everything and making a few units conversions I got the following:

1:8 twist rotational energy = 5.1 ft-lb
1:12 twist rotational energy = 2.3 ft-lb

Pretty much negligible compared to 1330 ft-lb. If your gun has 22 in barrel, the extra force on the bullet to get that extra 2.8 ft-lb of energy is 1.6 lb. It takes 124 psi on the back of a 0.224-in diameter bullet to get that force. Again, it's important to note I'm talking about the average increase over the length of the barrel and not an increase in peak pressure. Also, due to inefficiency, I would guess the actual increase in pressure to be somewhat higher. Still, even if the increase is on the order of 500 psi the change is less than 1% of 60,000 psi. Somebody with more experience than me can chime in, but I would have to wonder if a strain gauge on a pressure barrel would even catch that.

Long story short. The idea of fast twist = higher pressure makes sense intuitively, but the numbers don't add up.

PS Feel free to double check math, this was a rush job.

The Swedish judge gives it an 7.8 plus one bonus point for taking a poke at Ingwe!

Errrr... wait, in light of the topic of discussion and that Ingwe is involved, my phrase "taking a poke at Ingwe" does not sound quite right either!

John