If you press 'I' and hover over the guns picture it should pop up a tool tip telling you the type of ammo the gun requires. You have three inventory slots or containers for equipment left is uniform, then harness & then bag (if your unit is carrying one) click in the picture at the top to see what each contains (In Arma 3).
Arma 2 it's 'G' by default to open inventory & is a little simpler but nearly the same operation.
Arma 2 it's 'G' by default to open inventory & is a little simpler but nearly the same operation.