Drag the mag from the ammo container into the gun's mag slot (make sure it's touching the gun) then press "E" to load. Space to shoot, when out you can either reload instantly (Reread the first two lines) Or you can unload the gun then reload it (I do have an easy fix for this but I preferred leaving it in"
TLDR: E to load gun, R to unload gun (only if there isn't any bullets left) SPACE to shoot