AR magazine + give you a bronze throphy (10p on xbox) and a tactical perk in the game. It is unlocked by getting 80 kills from cover using the handgun.
This can be done by nearly anywhere in the game and there are countless opportunities. For example, in chapter 3, Oderbruche checkpoint, there are six enemies at the beginning of the checkpoint and of course you can restart it as many as you want.
To get the specific kill, take a cover behind a pillar, any chest high wall or crate, hold L1 (LB on xbox) and use the LS (L) to lean out and R2 (RT) to fire.
If you choose to get this trhophy with a double fire mode AR (i.e. the 1946 AR) remeber to select the fire correctly.