You need air vents at the rear so that the gas acts like a carburettor and draws air in with the gas.
You can't just rely on the air down the front of the barrel.
You also need a large washer type ring about 10" -15" from the rear of the barrel.
The breach end of my barrel is near 4" diameter.
The spark plug or what ever you use has to be fitted at the bottom of the breach as the propane gas is heavier than air so will mix with the air and be low at the bottom of the breach.
I recently purchased an electronic flysquater that I think can be used to ignite the gas ok if you pull it apart and wire it up.
An old cooker igniter will also do the job one that works off a 1.5V battery will be fine.
http://www.youtube.com/watch?v=_eh-CMAGbXY
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The 12 kit will work ok just a bit bigger than you need if you want to hide stuff.
No valve needed on the air intake.
The one on mine is like a 1"diameter pipe about 1-1/2" long and has 2 or 4 holes about 8-10mm diameter in the pipe. Start with small holes and make them bigger if you need to, hard to make a big hole smaller. 
As the gas blows into the barrel it pulls air in through the holes with it like a basic carb, if you hide it under the breach box you should get away with it.
Free speech is expensive these days!
The 12 kit will work ok just a bit bigger than you need if you want to hide stuff.
No valve needed on the air intake.
The one on mine is like a 1"diameter pipe about 1-1/2" long and has 2 or 4 holes about 8-10mm diameter in the pipe. Start with small holes and make them bigger if you need to, hard to make a big hole smaller.
As the gas blows into the barrel it pulls air in through the holes with it like a basic carb, if you hide it under the breach box you should get away with it.
Just watched the video again you can see the pipe is about 4" long and has 4 x 10mm holes near to where the gas is injected.
Free speech is expensive these days!
