main battery; main armament
Refers to the primary guns on a warship or the main firepower of a military unit.
The battleship's main battery fired all at once.
best hitter; center of the offense
Baseball term for the team's most powerful batter, often batting cleanup.
As the team's best hitter, he produced home runs in bunches.
The battleship's main guns roared to life all at once.
He excelled as the team's cleanup hitter.
They fired the main guns at the enemy ship.
The rookie player was entrusted with the cleanup hitter role.
主砲 refers to a team's top hitter or key performer, while エース more broadly denotes the best player, often a pitcher, on a team.
Compound of 主 (main) and 砲 (gun/cannon). The baseball sense is a metaphorical extension of the military meaning.