SpudFiles

ok im going paintballing this saturday (13th) and i wan t make an under barrel launcher for my tipmann 98 so it needs to be small but powerfull so i need some ideas i was thinking of one of those pocket cannons

sounds good. isn't Saturday the 18th. have you every build a co2 regulated pneumatic gun before. im asking because you posted in the wrong section, which means you're new the the web site and probably spud gunning. this seem like quite an ambitious project to put off in a week
edit: my bad, i heard paintball and assumed pneumatic.

yes i only make pneumatic guns this will be my first combustion

you'll need some sort of preloaded cartridge, I can't see you spraying deoderant into your chamber in the middle of a paintball game

i don't think this will work in time. for what you describing, it sounds like you'll need to use meter propane or MAPP gas. the smaller you go the harder it is to get consistent shot because its harder to get the right fuel to air mix. the only thing i think that comes close would be this http://www.burntlatke.com/mini.html

for a single shot put some flour in the chamber shake it up and ignite!No need to spray in fuel.

Dude, Flour? WTF?

Get a small propane torch, give the inside of the chamber a couple seconds from that and its all good. The torch pre-mixes the correct amount of fuel into the air (the holes at the base of the torch, thats the air intake). I made a pill-bottle (albeit a monster pill bottle lol) paintball combustion that could throw a paintball at 400 fps, though it was 3' long...

Perhaps coax would be your best bet for something like this, use 1.5" or 2" PVC with a 3/4" CPVC barrel (.69 ID ).

What you want to be practical in a game of paintball is probably something like my combustion cartridge, a unit that will take simple pre-filled cylinders with gas and ammunition - incidentally, what are you planning for the latter? Presumably a whole bunch of paintballs, so at least 1" caliber...

another thing is it needs to stay around 270 fps as a rule

With a combustion cannon its really hard to regulate speeds (unlike pneumatic) so perhaps sticking to pneumatics is more beneficial for this. If you build a pneumatic, it only needs about 50PSI to launch a paintball at 300fps. Therefore an onboard pump could work. I would make a piston valve too.

i can do that but it needs to be small max 5" long

Go for a coaxial. 3" chamber and 3/4" barrel. That would be your best, compact choice.

A coaxial would work perfect then.