Machine Guns
Light machine guns mostly have simple iron sights. A familiar aiming fixed order is to alternate solid ("ball") rounds and tracer explosive escorting (usually special tracer round for every four ball rounds), so shooters can see the trajectory and "walk" the fire into the target, and direct the fire of other soldiers.
Loading systems in initial manual machine guns were often Machine Guns from a hopper of loose (un-linked) cartridges. Manual-operated volley guns usually had to be reloaded manually all at once (each drum reloaded by hand). With hoppers, the rounds could often be added while the weapon was firing. This gradually changed to belt-fed types. Belts were either held in the open by the person, or in a handbag or box. Some modernized vehicle apparatus accoutrements devoted linkless feed systems however.
Closeup of M2 - This automaton gun is part complex armament subsystem; it is aimed and fired from the aircraft rather than directly
Closeup of M2 - This machine equalizer is cut composite armament subsystem; it is aimed and fired from the aircraft rather than directly