The best weapon is that it best suits the combat situation and copes with the assigned tasks. However, it often happens that what the military industry offers by default is, to put it mildly, not a very good solution. So you have to improvise. And about such non-standard modifications of weapons we will now tell.
Machine gun stinger
World War II, the guys from the US Marines Fifth Division, experiencing an acute shortage of firepower, take an AN / M2 aircraft cannon, attach bipods to it, butt from a M1 carbine and a homemade box magazine for 100 rounds, and go into battle.
Modular system Stoner 63 Modular Weapons Systems
One of the first relatively successful attempts to create a modular weapon. On the one hand, everything was fine – modifications of the weapon and a carbine, and under a standard rifle, and for installation on armored cars, and just under the usual light machine gun. Reliably, slaughter. On the other hand, the design turned out to be so complicated that the army command chose to leave the relatively simple and convenient M16.