4th, the original AR15s/M16s issued in 'Nam had 1 in 14" twist bbls. This was barely enough to stabilize the 55 grain ball ammo then issued. It would fly straight through the air but when it hit flesh, they instantly lost stability and started tumbling end over end, often bending into a bannana shape and breaking up. Think minature boomerang buzzsaw flying through you at 3,000FPS with little chunks flying off. This caused the terrible wounds that the higher ups thought would allow a small .22 to equal the performance of the .30 it replaced. All was well until arctic temp testing in Alaska revealed that the colder, denser air caused the bullets to destabilize in flight and tumble in midair. As a result, one of the many "improvements" to the M16 was a change to a faster 1 in 12" twist. No more tumbling bullets in the arctic, but now the bullet was so stable it drilled a 1/4 hole straight through people and got it's rep as a poor stopper.
The newer SS109 bullets are designed to break apart in the middle when they hit, which makes 2 wound tracks and of course the pieces tend to tumble. In theory anyway, a better stopper over the 55gr ball. So the US Army adopts that round as the M855, then turns right around and adopts the M4 Carbine version to fire it with a 1 in 7" bbl twist. Unfortunately, that breakup effect is dependant on velocity, so the shorter M4 with it's lower muzzle velocity results in less than spectacular wounding with the new cartridge. The Jarheads kept their 20" bbls though, and report generally good results. There is supposed to be a new M4 specific round out, the M855A1 that is a copper and steel lead free design. Haven't seen any field reports on it yet, might be the solution, might not.
In any event, if ya ain't in the military any more the Hague prohibitions on HPs no longer apply. There are some great hollowpoints out there that will get the job done.