Correct. Spray for about 2-3 seconds. Of course I don't exactly know how big your firing chamber is so you'll have to do some trial-and-error to get the result you want. You don't have to be quick about sealing up the chamber. good luck.
My latest cannon has a combustion chamber. I use 4 inch pipe about a foot long. An adapter down to 1 1/2 inch. A threaded end piece with a threaded cap holds the ignitor. I load from the muzzle, spray in the chamber, slap the threaded cap on, aim, spark...Click!...Fizz...Ka-whomp!!!!