It would be really nice if instead of an arbitrary limit on the number of guns, there was a more organic way of preventing someone putting 50 heavy cannons on their craft. In reality, storage space is one of the biggest limiting factor for a fighter's armament, but that doesn't really apply in KSP, so perhaps we could have very heavy guns/ammo instead so that putting too many guns on board will result in a plane that flies poorly. Cannon could be limited more by the weight and reduced quantity of the ammo as well as the additional weight of the weapon it's self..