I love how M107's look, nice pic for a design base....
When you sy mod a fire extinguiser do you mean to use it then like drill it out so you can fill it with compressed air?
If you plan on using a fire extinguisers built in CO2 I don't recommend that because it runs out fast and you can't refil easy (I don't think this is what you mean)
CO2 from a paintball cylinder is very easy to use... all you need is a reg that goes from about 800 to 100 psi... (Warning: CO2 in hot weather can get to 1800psi, so make sure your reg wont blow at high psi)
I have a fancy paintball gun

and my Max-Flo N2 cylinder is 4500 psi, but it has a built in reg that you can turn down to 100psi... unfortunately just the reg that goes on the tank is $150+ and the tank is $150...