Teemo poison shot works on gangplanks barrels

Gangplank barrels have 3 hit points but the poison part of teemo also damages 1 hitpoint allowing him to kill barrels when they have 2 hitpoints. This happened yesterday when i was playing gangplank into teemo.

In that same game a graves kept 1 shotting the barrels because apparently he can do that with his shotgun pellets. Each one of them acts as a separate attack and so he can kill 3 hp barrels with 1 auto attack.