SpudFiles

I don't know if there any real advantage over propane if you use air... its just a more inefficient engine.

Every time you convert an energy from one form to another you lose a great deal due to the inefficient transfer.

A liquid a fuel stores way more energy then any gas type fuel but if you insist... try using some fuels that people don't commonly use...

Now if you converted a regular engine to run of MAPP Gas or acetylene that would be fun...dangerous but fun to see the results

most people voted for me to make the engine but after what people have posted on it, I don't think I have the experiance or tools to do it. I am leaning toward a large pneumatic gun or make my first attempt at a hybrid. What do you guys think?

I say go for it. I'm making a large pneumatic my self and the potential is huge... Again make something that you are comfortable making that is with in what you think you can build

Honesty hybrids make me a bit nervous thats why I've never really attempted to make one. I've always been able to get all the power I could use out of my pneumatics

Make the hybrid then you will have so much more fun shooting at random stuff than with the large pneumatic.
or you could combine the best of both worlds and build a large hybrid