Do not make it like a potato gun. Do not use combustibles.
You can get plenty of range by using a ball or sprinkler valve and a bike pump. It is safe and outdoor friendly using pneumatics.
Also, if you wanted to make the process just a tad bit easier, once you have it set up with a shrader valve (bike tire stem thingy that you use to pump it up), you can use a CO2-powered quick-pump to fill the chamber. If you are THAT lazy.
And there's always room for improvement!... and increased cost. Use solenoids and a small electrical system to control the airflow from the comPRESSion chamber (not combustion!). Add a large, regulated HPA or CO2 rig(think... paintball gun tank) to the device for multiple shots (if you carry multiple projectiles). A 12oz CO2 tank (pretty small) will give you about 20-30 shots to some pretty huge ranges.
Then put this whole thing into a deactivated TOW or AT4 tube for the realistic look to add even more cost and cool-ism to it.
i remember watching a series of videos on the net with something similar to what you described, the guy used a regulated paintball tank and got off a decent amount of rounds per minute.. so it most definitely can, and has been done before.
i believe he used a modified sprinkler valve, PVC, pvc glue, JB weld, and a shrader valve.. i saw it over two years ago though so i'm sure i forgot a few of the essential parts.. the construction was simple enough, the key seemed to be keeping compression
edit: my apologies for the continued necro