Occasionally magazines for any gun can't be loaded with the correct ammo, the loading circle will come up on the screen but immediately cancels. Unsure if it is the ammo stack or the magazine itself that causes the issue but neither of which are damaged, it just bugs out and won't allow you to fill it with ammunition. I think it is related to the inventory issue where you cannot take certain items from the inventory or items in hands show up differently to your friends than what you see in your own hands.