I notice with some ammo on some maps on the ground you can't pick it up because you don't have a gun that goes with it
I think there should be something similar for the F2 shop, because it can be infuriating when you buy ammo for the wrong gun type, for example: when you're trying to buy 5 boxes of 5.56mm ammo but end up buying 5 boxes of 7.62mm ammo, can be annoying even more so when you don't even have a gun that takes 7.62mm ammo.