If you make them in a small batch of say, 12 cartridges, you can cut the time down to about 15-20 minutes per shell (3-4 hours total), depending on how much detail you want to put into them.
Step 1: Making the Tube
1. Find some 20mm diameter PVC tube, and cut it into 70mm long sections. If you can get the right colour tube that you want, great! If not, get it in white, and obtain spray paints of the correct colour.
2. Use either a craft knife, or stepped drill bit to bevel the edges and remove any rough edges (Picture 1). It's also a good idea to sand the ends down.
3. (Skip if you already have the right colour tube). Spray paint the tube sections. I hung all mine from a piece of sting between 2 chairs. make sure to spray inside the "opening", although the other side doesn't matter (See 2nd image).
Step 2: The Metal Caps
Next, draw around the end to prodce a circle on the card. the circle you draw will be about 1mm wider than the diameter of the tube, but this will be negated by the thickness of the card. To create a metal "lip", make sure that when you cut out the circle, the scissors stay about 0.5-1mm outside the pencil mark (Which I forgot to do). You'll get an idea of the necessary width after making your first circle.
Step 3: Add Aluminium Foil
Step 4: Attaching the Cap to the Tube
Apply more glue to the rim of the tube, add the foil/card circle to it, and apply some weight to the top end
Note: Your choice of glue may dissolve the paint, or leave a residue on the plastic. If it's that sort of glue, use as little as possible, and clean it off your fingers where possible.
Step 5: Usage / Extra Details
My use for these is to complete the Collar/Breacher parts on the torso and wrist attachments of my Halo Reach armour.
Hope you enjoyed this instructable!
Step 6: [Updated] Actual Cartridges
The most iconic part of real cartridges is the detailing on the metal caps, so if your only option is to make them yourself, it's well worth engraving a few letters into the foil. The grooves down the plastic also add realism.