What it does
"Battle Cannon" isn't a standard Magic keyword or ability—it doesn't appear in the comprehensive rules or on any official card. It sounds like it could be flavor text, a homebrew/custom card ability, or a misremembering of an actual mechanic.
You might be thinking of something like "Myr Battlesphere," which creates Myr tokens and can tap them to deal damage when it attacks, or perhaps a Universes Beyond card (like Warhammer 40,000 Commander decks) where flavorful named abilities appear. Cards from those sets sometimes have evocative ability names, but the actual rules text is what governs play.
If you saw "Battle Cannon" on a specific card, check that card's reminder text or Oracle wording on Gatherer or Scryfall—that's the authoritative version. If it's a custom card from a playgroup, ask whoever made it to clarify the intended rules, since homebrew mechanics carry no official ruling.
Iconic cards with Battle Cannon
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