I’ve noticed in some aircraft (I play mostly arcade) that have two types of machineguns, or machineguns and cannon, such as the P-36 or the Bf-109 E3, sometimes (quite frequently actually) one of the gun types will stop firing entirely. It is not jammed. Reloading the weapons and even landing at the airbase does not fix the issue. In fact the weapon’s ammo shows full and it does not reload when you hit R.
You selected a specific weapon?