muzzle; gunpoint
Refers to the front opening of a firearm barrel. In contexts like 銃口を向ける, it can mean 'pointing a gun at someone'.
犯人は店員に銃口を向けた。
The criminal pointed the gun at the store clerk.
銃口から煙が出ていた。
Smoke was coming from the muzzle.
muzzle flash
Compound of 銃 (gun) and 口 (mouth, opening).