As far as I can determine from those ballistics tests I can find online, the M855A1 (and the
CBC 5.56x45mm SAT Improved Performance, which is a Brazilian-made equivalent) penetrate as AP in GURPS. That is, they penetrate steel far better than M193 or M855 rounds, even better than M80 ball in 7.62x51mm,
but not as well as APHC rounds like the M995.
Unless the terminal ballistic tests in gelatin are unrepresentative of actual performance, however, the M855A1 does not suffer reduced wounding for being AP. It should perform similarly to M855 from 20" barrels from the 14.5" barrel of the M4, even possibly somewhat better.
Personally, I don't see any harm in assuming that Mk318 SOST, M855A1 and a number of advanced rounds being adopted in the 2010s simply represent a TL advantage over typical TL6 and TL7 FMJ and AP rounds. CAD and CNC technology, ever improving propellants and the use of more expensive materials than simple jacketed lead ball allows optimizing rounds for their intended role and improving penetration doesn't automatically reduce wounding.
The published performance of M855A rounds seems to justify Dmg 2d+7(2) pi+ from 14.5" M4 barrels, though at longer ranges, the pi+ would be reduced to pi (as carbines in ordinary 5.56x45mm ball become pi-).