I think this isn't enough drawback. It's hard to deal damage to a 4/2 double strike, let alone if it flies. I think this card needs to change.

How about "Whenever equipped creature deals damage, destroy it." ??? The cannon is so overcharged that the equipped creature gets caught in the backblast and fried?? (brings up very goblinesque imagery this way)

Heck dropping it to +1/+0 and Double Strike still makes it potent (2 dam extra total rather than 4) and could then possibly justify the drawback as is.

Heh. "Whenever equipped creature deals damage" is awesome, but it makes the double strike somewhat less useful, as it'll die after first-strike damage and before normal damage... :) "Whenever equipped creature deals damage for the second time in a turn", perhaps? It's somewhat fiddly wording - no longer anywhere near as simple as it currently is - but a bit more fun for Johnny...

Taking it down to +1/+0 is definitely one reasonable option. I'll see if I can come up with any others.

how about "deals damage sacrifice it at the end of the combat phase"??? gets around the double strike issue quite cleanly.

That's another sensible suggestion, although it works rather more oddly with noncombat damage (such as from Heavy Arbalest) than the current way does. I used to have a deck with Lowland Basilisk + Hermetic Study: the pinged creature would die at the end of the next time there was an end-of-combat step... which could be several turns down the line!

I like the funky wording right now. Gives me a little mini-game to make sure I kill a creature with only first strike damage, so that my creature doesn't die.

Yay, cool. That's what I was hoping for.

Ignoring the fun minigame for a moment, this is now an interesting comparison with No-Dachi. It was accidental that the costs match so precisely, but it's probably okay given this is rare and that was uncommon.

If it kills all creatures attacking/blocking in the first-strike damage step, does it then not deal damage in the regular combat damage step? (Which is fine, I think that's entertaining and dangerous enough.)

Indeed it doesn't. Unless it also has trample, if this kills everything with the first strike then it doesn't explode in the normal damage step.

Of course, it's possible this looks more fun than it is. It may just be "equipped creature gets +2/+0 and double strike and is unblockable. Sacrifice that creature at end of combat." Oh well, that's still reasonably interesting, and the moments when it's different from that will still be fun.