A Great Topic !
HOW you do it is with an Electric Match. They are made for the fireworks industry, and you can buy them online @ eBay, or PM me and I can get you some. 'E -Matches", as they are called, are two 22ga wires that are typically 3M long, and terminate in a 4MM square x 1MM thick mount for a super-fine nichrome wire that spans a tiny fork. The end is dipped in Nitrocelluose Lacquer, then a sealant.The end is the size of a strike-anywhere match. Every major fireworks show has hundreds of these, and in bulk they are about 55 cents each. 3 Volts will fire them nicely, and they fit down a vent hole like magic.
I like them for cannon firing because they give complete control over ignition like a pull fuse igniter, but cost less, and there's no physical movement to change the point of impact with a smaller cannon. I fire mine with a modified claymore clacker, and it works like a charm. I don't have an ATF opinion letter on these, but if someone does, please post the response.