What it does
I can't find "Rapid-fire Battle Cannon" as an official Magic: The Gathering keyword or ability—it doesn't correspond to any real game mechanic. It may be a fan-made card, a misremembered name, or something from a different game entirely.
If you encountered this on a card, double-check the spelling or the card's actual rules text, since the gameplay effect would be defined there rather than by a keyword. A few real abilities sometimes confused for invented ones: Bushido (creature gets +X/+X when blocking or blocked), Double strike (deals combat damage twice), or activated "tap to deal damage" pinger effects that resemble a "cannon."
If you can share the full card text or where you saw it, I can explain what it actually does and how it plays in Commander.
Iconic cards with Rapid-fire Battle Cannon
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