In an M16 barrel with typical military internal surface finish with chromed bore, soft-swaged commercial HP match bullets, regardless of weight, will start coming apart in the 7" twist A2 barrels unless you keep velocity under about 3000 f.p.s., as the thin gilding metal jackets cannot stand the high rotational stresses and the rougher surface finish of military barrels causes longitudinal stress risers by scoring the jacket and giving it a head start to coming apart. In production grade barrels I wouldn't go faster than 9" is I needed a barrel suitable for either M855 ball, 69MK or commercial 55-gr. softpoint or varmint loads with 52-53-gr. JHP bullets.
With smooth match-grade custom barrels having an internal surface finish of better than 16 microinch root mean square, the better custom barrels buttoned are about 8 RMS, and the high dollar, hand lapped benchrest or electropolished premium hammer forged GFM stuff about 4 RMS or 1 micron, you can go as fast as 7" twist and the 52-gr. bullets will stay together and give good accuracy, even with the light bullets, because the jackets are not damaged.
I had an Obermeyer 7" twist 5.56mm pressure test barrel blank chambered in .22-250 and fitted to a Hart 1A heavy varmint action which would shoot bugholes with good Berger or Culver 52-gr. handmade bullets all day long and under 1/2 moa with ordinary store bought Sierras with either 38 grs. of H380 or 36 grs. of 4064. I'd use the Hornady 55-gr. SX or Sierra Blitz bullets for groundhogs in this rifle and they'd give a blue smoke trace all the way to the chuck and blow them up like dynamite! Wouldn't exit a chuck at 300 yards. It was like picking up a gurgling leather bag with 8 pounds of strawberry jam inside. No cripples, no riccochets.
