SpudFiles

So I'm building my first gun. I need some technical advice. The guns barrel is 16 in long, 1/2" bore. How long would a 3/4" combustion chamber need to be? Also with all the PVC fittings, will the 3/4 inch chamber fit in 1 inch tube?

.8:1 chamber to barrel ratio is commonly accepted as the most efficient ratio. So that would mean you want the chamber to be 80% of the barrel volume. Again the chamber volume should be slightly less the the barrel volume.

A larger chamber will produce more power but it explosion won't be used to its full potential . Click on the button towards the top of the page that says wiki. Then click on combustion launchers. Lots of great information and ideas there.

Thanks for the ratio. Got my dimensions down. Now as for PVC fittings, will the fittings for 3/4 inch PVC fit inside 1 inch PVC pipe?

No. You want a bell reducer which should be less then a dollar.

Just for clarification, I must make it clear. My plan is to have the 3/4 inch combustion chamber contained within a larger tube. 1 inch if possible, 1 1/4 most likely. This is more for aesthetics than anything, but also serves as extra protection in the event of a failure.

Meh. I don't even recall a conventional combustion launcher that failed at the chamber wall. A improper joint seems to be the main mode of failure. Short socket depth, lack of glue, loose threaded connection.

What im saying is, just buy some decent pipe. No need for two layers. you want it to be pvc and schedule 40 or thicker.

It is what it is. The two layers are more for concealing the cannon within a streamlined design. I mentioned the other part because I like a bit of redundancy when it come to explosions near my face.

Progress has been made. I finished the cannon this afternoon. Its ready for its first shots. I have an 18 inch barrel of 1/2 inch PVC and a 6 inch 3/4 inner dia. combustion chamber sparked by Char-broil brand BBQ igniter. All of this concealed in a 1 1/4 inch PVC upper receiver and a lower receiver taken from broken Walmart H&K P30 Airsoft pistol. The barrel is threaded onto the chamber because with the stock in place there is no way to access the combustion chamber.

Bravo! Looks good and realistic. Kind of reminds me of the bizon smg. I bet you would take an interest boltsnipers builds http://nerfhaven.com/forums/index.php?showtopic=6183

I suppose the greatest question of all, how does it shoot?

I have yet to find out myself. I was performing testing last night and my igniter began to fail. Less than 40% of my trigger pulls actually generate a spark. I am also having fueling issues. And as for boltsniper, I already took interest in his builds. In fact my next cannon will be partially inspired by one of his pneumatic guns. I simply can't decide on weather or not I want to make it an Airsoft gun. LOL