Reply with quote #9
When we did the 20 Practical project AR rifle, it was felt that if you go with the heavier bullets (like 50+ gr bullets) you might as well just shoot a .223 and not bother going with a 20.
With the 40 gr or less 20 cal bullets, you can run with more relaxed twist (less pressure) like a 1:11" twist that allows you to shoot the bullets very fast and flat. When you get up to the 50+ gr 20 cal bullets, you have to go with a much tighter twist (pressures rise) and it's hard to get the velocity up much more than you can do with a 223 shooting the 50 or so gr bullets.
The whole point of the 20 Practical is to take advantage of the niche it has to offer, not to try to make it do what a 223 already does.
With the 40 gr or less 20 cal bullets, stick with a zero free bore, as you need to do that to keep close to the lands and still have a reasonable amount of the bearing surface of the bullet in the neck of the case too.
Watch out for reamer designs too. There are some 20 Practical reamers out there that are definitely no good for running out of a semi auto AR-15. There is a definite difference on the reamer designs that are appropriate for running the 20 Practical out of an AR-15 and the reamer designs that are o.k. for bolt gun use (been there, done that).
Reply with quote #10
Appreciate the advice.