Federal XM193 ball. Fragments easily, and tends to not be overpenetrative. While I know lots of folks like soft tip or plastic tip rounds for the AR just to be safe, it is not needed. The main wounding mechanism for the 5.56 cartridge is velocity, and when you use a shorter barrel then 20", you lose the original 3200 FPS. Carbines w/ the 16"barrel(or14" GI) suffer roughly a 400 to 600 FPS drop in speed w/ XM855 ball ammo. That combination can and does create wounding problems. Case in point, Battle of the Black Sea(Mogadishu fight). W/ an M4 and M855 ball, the round tends to ice pick human targets w/ little to no serious wounding happening.
The M193 round that was fielded w/ the original M16 in Vietnam proved to be a serious anti personnel round, but showed poor penetration thru barriers. The Viet Cong called the M16 the "Poison" rifle, as personnel shot with it were seriously wounded and often succumbed to their wounds at a later time. IIRC. This was due to the M193 round moving at high velocity and fragmenting quickly upon impact.
If you use a 16" carbine, use of XM193 ball will bring your velocity back up somewhat(2800+ FPS), and due to bullet design, give you a round that tends to fragment easily upon contact w/ targets. Downside is most 16" carbine's have the 1 in 7" rifling twist designed for the M855 ball round, and sometimes accuracy suffers when using XM193 rounds.
The difference between the M16A1 and A2 rifle(and susequent M4) and required ammunition for both has created a holy mess as to what ammo to use with what rifle.
When I was at Benning working Range detail, I could see where the KD range Plastic pop up targets had been hit with a M855 ball round, fired by an M16A1(I was there during the transition of rifles). Due to the 1/12" rifling twist not stabilizing the M855 ball round, you could see where the longer 62 gr bullet had slapped thru the plastic sideways, due to instability. And this was at 100yds. Thank You, my beloved US ARMY. What a Charlie Foxtrot!
Sorry for the long missive. It is a subject in which I have always had a great interest in. As always, YMMV. :supergrin: