automatic firearm
Technical term for firearms that automatically reload and fire continuously while the trigger is held, such as machine guns and assault rifles.
Possession of automatic firearms is strictly regulated by law.
小火器 (small arms) is a broader category that includes non-automatic firearms like pistols and rifles, while 自動火器 specifically refers to automatic weapons.
Compound of 自動 (automatic) + 火器 (firearm). The term is a straightforward technical compound with no obscure historical derivation.