FMJ stands for Full Metal Jacket – basically it means you have more bullet penetration – so you do more damage through walls etc. I know from talking to players on Xbox Live that this isn’t a particularly good attachment, people are struggling to do the 40 FMJ kills challenge to unlock the extended mags – the reason? It is just a bit pants to be honest, better off levelling up the next gun instead of wasting your time with FMJ.
Update: to get the FMJ challenge, it counts if you shoot them through a window – shouldn’t take you too long now eh? 
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