You could try removing the safety switches. One of them could be not quite pressed down all the way or just damaged. Either way it would be a good idea to get rid of all the unnecessary components for testing.
I would cover the stryfe in modeling clay and build the shape and details with the clay. Then I would build a mold using the clay model and plaster. Next up would be to cast a fiberglass shell inside the plaster mold and attach it to the stryfe.
I second Draconis given the additional information. A flywheel blaster would be the best bet, that way the tampon diameter wouldn't be such an issue. You may need to replace the original barrel with something a little larger but that's not really such a big deal. If your only trying to launch a projectile 6ft or so into a crowd of spectators I doubt accuracy will be an important issue. Although you may want to increase the voltage going to the motors to give it a little more distance. You will most likely need to modify the magazine of the blaster you will be using so that the tampons won't jam or get caught.
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