Bullet concentricity and twist rate

[Bart B.]

Spitballer,

Never crimp your reloaded ammo if best accuracy is your objective. Crimped bullets require more pressure to push them out of the barrel. Commercial match ammo doesn't have crimped in bullets because they know it hurts accuracy. It also increases the spread in neck grip on the bullet. And bullets are never crimped exactly the same all the way around anyway. Where the bullet is when the pressure peaks doesn't matter. It'll be in the same place with the same load anyway. Repeatability is the foundation of accuracy.

The best neck grip on the bullets is the least amount needed to hold them in place until they're chambered. That minimizes the release force spread needed to push it out; muzzle velocities are more uniform and so is the bullets time in the barrel.

If you ammo's properly loaded, the difference in accuracy across several distances of bullet jump is small. Best example I know of is wearing out over a dozen barrels shooting 308 Win ammo and bullet jump distance to the lands increased almost .080" over the 3000 + rounds of accurate barrel life.

If you insist on a given jump distance and you're shooting a .308 Win, the origin of the rifling where the bullet touches it advances about .001" for every 40 rounds or so fired. Are you going to change the bullet seating position for each 40 rounds of ammo you use?

 

[RecoilRob]

Our M-16A1's had 1:12 twist and would barely stabilize the M193 at 500 yds when it was cold and damp. They shot well for most of us at 200-300...but darn if they didn't start to get a bit wild at 500 and I found out later it was the marginal twist rate. The 5.56 should be able to shoot out to 500 with proper bullet and twist selection as has been proven many times but I thought I just sucked at 500 yds. (probably true too)

The least amount you can spin the bullet will leave the most potential energy from the powder charge to develop velocity rather than bullet spin. Each round that leaves the barrel carries with it the same potential...but the kinetic energy of the velocity will be reduce with a very fast twist compared to a slower one as it takes effort to spin up the bullet. The fast twist barrels are turning the bullets over 250,000 rpm and if a lightweight bullet is fired at too high velocity they can rupture from the gyroscopic forces so using no more spin than needed for the stability you need at the range you're shooting is a good idea. 1:7" barrels and 45 grain varmint pills sometimes leave a grey puff in the air part way to the target...and no holes, so do choose bullet weight to be compatible with the barrel and velocity to be used.

 

[Bart B.]

Never heard of this:

The least amount you can spin the bullet will leave the most potential energy from the powder charge to develop velocity rather than bullet spin. Each round that leaves the barrel carries with it the same potential...but the kinetic energy of the velocity will be reduce with a very fast twist compared to a slower one as it takes effort to spin up the bullet.

Does the fact that a 22 caliber barrel with a 1:8 twist has the same rifling angle to the bullet as a 30 caliber 1:11 twist have anything to do with this?

Please show an example.......

What's the formula for calculating downrange velocity kinetic energy that uses twist rate as a variable that subtracts from it?

On another note, it was interesting what Frankfort Arsenal did with the military 30 caliber 150-gr. bullets' spin rate when developing the 7.62 NATO round. .30-06 150's leaving at 2900 fps from their 1:10 twist barrels were not all that accurate. So they used a 1:12 twist shooting the same bullet weights out 100 fps slower and got much better accuracy. M14 barrels at 22 inches had a 1:12 twist. Same twist shot the 172 grain match bullets more accurate from the NATO case than from the .30-06 case 100 fps faster it its 1:10 twist barrel.

The icing on this twist cake was what the British Empire's fullbore long range competitors did with their 7.62 NATO chambered rifles. M80 ammo's 147-gr. bullet wasn't shot fast enough in standard length barrels so they used 30 inch ones to get another 100 to 150 fps velocity to stay supersonic. Then changed the standard 1:12 twist to a 1:13 to get better accuracy. In hot weather, some used a 1:14 twist for best accuracy at the longer ranges.

 

[spitballer]

Are you going to change the bullet seating position for each 40 rounds of ammo you use?

...yes, I'm out a couple thousandths already and this is what led to all that checking and re-checking the other day! I'm not eroding quite as quickly as you've suggested, but it's obviously a dynamic that I'm going have to stay on top of. Seating against the lands with a lot of neck tension is difficult to repeat accurately so I've again ruled this out, but based on past experiments I'll stay with a closed gap which brings me back to option A). I agree with our esteemed moderator that it's going be hard to stay at .002" but I'm going to try

Also found RecoilRob's comments about stabilization very much worth reading. Frankly I don't think he and BartB are actually very far apart on this. Looking forward to a next .223 barrel I'm considering 12", 13", and 14" twists which will produce stability factors (per JBM) of 1.46, 1.25 and 1.087 respectively. Theoretically the 14" twist will stabilize at 1.087, but does anyone have experience stabilizing a bullet with such a low margin of error?