Extended mags are normally unlocked from completing the FMJ challenge (40 kills I think). It is very hard to unlock, but extended mags is actually probably quite useful – it will prevent you having to reload as much. Although for some people who obsessively reload, this attachment has no advantage – but for those who like to spray or use cover fire or shoot through objects, this attachment should be particularly handy.
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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I’m trying to unlock the mags for the Scar-H, i think extended mags will be very handy when i’m trying to get my killstreaks up. 40 FMJ kills is proving to be quite time consuming though!
To get the FMJ challenge, shoot them through a window – it counts a shot through an object, much easier
i have nearly unlocked extended mags on my m4a1 it is a good gun with FMJ but i am going to use the acr on a lot of free for all matches.
I am going for extended mags on the M16 right now, already unlocked it on the SCAR. It definetely takes a while and I will probably not use it but I like the big bonus I get for getting all attachmets unlocked for the gun.
To get the FMJ challenge, it counts if you shoot them through a window – shouldn’t take you too long now eh?
Have been at it for a while now, have unlocked everything else and have pretty much completed all the other challenges for Scar H but this one is holding me back. . .Getting there slowly but surely
Highrise is a good map to pump up ur FMJ scores the black glass can be shot through soo u just put one man army and spray the heck out of their spawn!!! i got a 30 killstreak with that…sadly i didnt hav my tac-nuke on…