Gun magazine + give you a bronze throphy (10p on xbox) and a tactical perk 4 in the game. It is unlocked by getting 15 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.