Hey all I'm a first time poster, but I've been browsing through for information for quite awhile.
I'm a combustion enthusiast, and have been looking for a better, more controlled and adjustable ammo.
Now this has been mentioned before, but I have found paraffin wax (found at your local grocery store) to work the best.
Here are the steps I follow to make batches of 9 rounds at a time for a 1.5" barrel:
Cut up a section of PVC, for the best balance, make the sections as long as they are wide (example: 1.5" by 1.5").
Seal off one end of each section. I like to use no-stick foil, sealed on the sides by a .5" wide strip of duct tape
Put the sections open end up on paper plates
Take 1 lb of wax and melt it in a double boiler
Pour the wax into each section, place in the freezer and wait 10-20 minutes.
The best part about these is when the wax contracts when it freezes, it creates a concave indent in the rear, making it slightly front heavy so it flys well, and the diameter does not contract, so the fit is just about perfect.
On top of that, just about anything can be added to the wax before it solidifies. I can fit 8-10 paintballs in a slug, leaving an impressive mark on my target (plywood board, don't worry!). If you want more weight, I've found adding bb's, steel slingshot loads, lead weights, etc. work very well.
With my 4"x9" chamber propane powered gun, I can plug holes all day through 3/8" plywood. By adding some weight I have been able to shoot through 1" plywood!
Again sorry if this idea has been presented before, I just did not see any detailed write-up on it.