SpudFiles

Im in the process of building a Advanced Combustion Cannon that will shoot marbles that are around .65 in diameter. The Combustion Chamber is 15 inches long by 3 inches in dimater. The Barrel is 60 inches long by .75 in diameter giving me a 1:1 ratio . It will have a 2.5 inch computer fan inside, that will run off 12 volts. Im still building the meter for the mapp gas. Ill have cam couplings so I can split the barrel and put a burst disk in. ( what kind of material will hold to 45 psi ?) A guy ran the specs into a Hgdt program and it said it would shoot in the 1100 fps with a burst disk that held to 45 psi. Im not expecting that much fps, I'm hoping for around 850fps+. Heres my main question, if the projectile, a marble for example isnt the exact size of the barrel, how would you get a good seal ? Or do you have to find the right size barrel ? What size of Barrel is required to shoot marbles, I know marbles come in a few sizes?.

Thanks, I just need some ball valves and Ill have some pics pretty soon

Normal glass marbles that you would find in the hobby section at Wal-Mart fit near perfectly in 1/2" type M copper tubing. A few to the majority of them actually won't fit, depending on the lot you're buying from, because the fit is so close.

Well, persuming you mean the barrel is too big for oyur projectile i am pretty sure you can't get a good fit with a barrel that is too big(really?). I soem how(totally) doubt you will get any where near the velocity you are expecting with 45psi. I suggest downloading HGDT your self and finding out. So to sum things up:

1. Find a barrel with a good fit
2.Get HGDT and find out if your figures are correct.

The only other alternative to get a good fit would be to sabbot the marbles or to wrapt them in duck tape until they do fit.

Hi,
850 fps is pushing it a little with a combustion, look at Chaos' pneumatic gun, he pressurized it to 400 psi and managed to get a marble to 900 fps . I'd say 400 if you're lucky. A .65 marble weighs just over 5 grams on average.
To get a tight fit, there are two main options I can think of : first a sabot of the exact (or close enough) diameter of the barrel. Second : some sort of wadding...

Lol I wish I could download hgdt