I am not sure what you would use for the amount of propane. I am leaving that for somebody else to figure out.
The actual combustion chamber is much smaller than actually shown. It would probably be just be the length of the reducing fitting. And as for the spark I was thinking of something with a continuous spark. I forgot what kind of device creates this, but I know I have heard about them.
You could use a fairly long barrel to compensate for the inefficiency.
Oh, and for triggering the burst disk, I was just thinking about having the compressed air burst it. I now realize that because the burst disks may not always burst at the exact same PSI, that the propane/air ratio will be messed up. Maybe a triggered burst disk, or maybe just a piston valve, or something of the sort.
Forgot to answer question on burst disk.