updated, vector version of my favourite small arms design.
NATO 5.56 Assault rifle, loaded from a 50 round horizontal mag above the barrel. Breech is above the pistol grip, ejection behind it.
To load, the magazine is released by pulling back the rear sight unit firmly, which unlocks the upper dust cover, spring-loaded and hinged at the front, and pushes out the spent mag. The new mag is seated in guides in the dust cover, and locked down into the breech. As shown here [link]