Nah I just read online that the FMJ attachment is exactly like the Deep Impact perk from COD4 (more penetration through walls and stuff). I got fooled by the choice of words that described the FMJ in-game.
I've also done some testing with it too. The dmg of the M16 is almost pretty much full to begin with. Add Stopping Power and FMJ and it should be an automatic one burst shot kill, right? Nope, wasn't the case for me as it still took two burst shots to drop someone, no different than an M16 with only Stopping Power, which I've been using for a few days.
Btw, MGaborik, I'm the ViceKingz guy. Like I said, I'm the one with the winking girl avatar lol