IIRC the A1 actually ditched the tungsten core, it's a "lead free" round not AP, the M855 was supposed to be an "environmental friendly" round the tungsten isn't a penetrator really it's some sort of composite matrix but after studies showed that Tungsten is just as bad an even worse than lead as far as the environment goes they've redesigned the round.

The M855A1 is also lighter than the M855 so if my memory serves me correctly they've switched to a mild steel core.

The M955 does use a tungsten carbide penetrator and is classified as an AP round but the usual "green tip" (or well just "green" ammo now since the A1 doesn't have a tip color) ammo that is in use uses steel penetrators for the most part.