I build my own ARs. It is very difficult to get barrels with the original twist rate of an M16. By original, I mean an M16A1. (it is also very difficult to get ahold of barrels of any twist rate today.) I have barrels of various twist rates. A faster twist rate will stabilize a heavier projectile, so it is needed for loads like a M885 or heavier. I also have loads that use 40 gr. projectiles, much lighter than a M193. What I am going for in a lighter projectile is a very high velocity for the purpose of penetration of body armor, not fragmentation. It is a different approach than green tip penetrators (M885). I picked that up from a guy I know who works for the Secret Service and buys his reloading supplies from the same gun shop that I do.
I did a test on bulet penetration.Loaded the M193 bullet in a .22-250 @3650fsp.I used old John Deere plow shares for the target,very tough steel.THe M80 7.62 load when fired from my HK 91 would not penetrate,sometimes the plow share would be broken in half -but no penetration.The M193 bullet blew 3/8 inch diameter holes through the plowshare every time.Looked more like a hole made by a cutting torch.