How does Doomguy add ammo to his weapons? – From /r/Doom

Especially in the new games since they removed reloads (except the ssg in 2016 and eternal) so how does he add ammo? Like the weapons have no ammo port at all or is probably unseen the new ssg in the dark ages is lever action, so where’s the loading port? I mean the shotguns are understandable cuz their loading ports are probably underneath the gun but look at the heavy assault rifle from doom 2016. Its “magazine “ is part of the gun. So how does he get the ammo in?