"spark from primer + gunpowder = the gas it gives off."
Uncontained smokeless powder is NOT and explosive but it does burn fast and gives off large qualinties of gas. Unless your school lab has a specromerter, keep the question simple. Perhaps limited to the actual volume of gas produced, as Mr. Patterson suggests.
You could then use the gas laws of physics (Boyles Law) to compute the (approximate) pressure that would be produced inside the small volume of a brass cartridge case, which you would also have to measure. Do that with water, in CC, converted to cubic inches if you wanted to push it. You won't be totally accurate but who cares, it will still sound good for a school project!
SO ... you do NOT want to use a large quanity of powder, or your principle will panic. Use perhaps no more than a level tea spoon full, uncontained except for a large bowl or something like that for a burn chamber, and perhaps use a hot wire for ignition. I suggest you try to capture the gas produced in something like a large (deflated) balloon. Measure the resulting volume and do the computations. If you don't know how to do the volume vs. pressure calculation, contact your schools physics teacher, he will likely be glad to show you how to do it.