Some missions contain stationary weapons amongst the ranks of the enemy. These are particularly dangerous targets, their firepower can take you down very quickly if you’re not careful.
Although the guns themselves can be destroyed, it is usually easier to eliminate their operator. This is difficult to do from the front though, you’re better off flanking the position or attacking from the rear where possible.
Once the operator is killed, the emplacement can be taken over and used against the enemy. Move close to the weapon and press the F key to operate it.