I was just going to mention that Ron; the fin can has a bulkhead capping the coupler. What I would suggest to you is to ground test your altimeter. You can do this with any old wide mouthed bottle. Some of these new sport drinks have such a bottle, as do bicycle water bottles and some others found in housewares at your local store. A piece of aquarium hose or R/C Airplane hose can be used as a vacumn hose. Drill a hole in the lid for the hose and one for ignitor wires.
You can use cheap Estes ignitors for the test so you can save your e-matches for the launch. Remember that, do not try to hook up ignitors to the altimeter as it will probably not fire them. Hook up the outside wires to a couple Estes Ignitors or matches, then turn on your altimeter, drop it in the bottle and tighten the lid. Suck on the tube as much as you can to create a vacumn and hold it, watching for the first match to light, slowly let out the air to see if the second match fires and your ground test of the altimeter is completed.
Make sure the pistons move freely inside the bodytubes. Sand them if they don't...they should be able to drop in the tube smoothly under their own weight. To strengthen the pistons, I "double walled" them, using a 4" PML phenolic coupler I purchased extra. I cut it lengthwise, then wrapped it around the inside webbed cord and squeezed it to fit inside the piston. I then cut it to length and epoxied the second "Wall" inside the piston. One coupler was enough to do both pistons. This will make it much sturdier and allow you to sand more if a better fit is needed.
Use around 3/4 gram of black powder in the lower/apogee section and around 1 - 1 1/4 grams in the upper section for the main chute and the rocket will seperate fine.
Good luck and keep us posted.
Carl