1.
cannon, artillery piece
A large, heavy weapon that fires projectiles. Refers to historical cannons and modern artillery.
2.
big hitter, power hitter (figurative)
A person with great power or impact, especially a powerful batter in baseball. Used as a metaphor.
Usage
Sense 1 appears in historical, military, and museum contexts. Sense 2 is very common in baseball commentary, where 大砲 refers to a slugger or power hitter who can hit long balls.