If you haven't already read the excellent replies to my "barrel tuning" post - I hope you'll take a look at it; good dope there.
That said, in the post I mentioned tuning a Big Bang Cannon for even louder sound. I'd planned on doing some tests before posting, but I'll share a recent idea I had for testing loudness; my Radio Shack SPL meter only goes to 120db, so I gotta fix that!
I have a decent selection of large-ish diameter tubes with thin or thick walls, long and short `uns. I'm thinking of simply sliding a particular diameter tube over the intactcannon, at a set length, then fire it per the instructions - it's safe since I'm NOT substituting barrels -- just seeing if an 8" dia. tube x 2' long for example would enhance report. If I don't get satisfaction, I'll consider setting up the same type of test, only without the factory barrel, and rig a lanyard firing mechanism for safe distance testing. (I live in Palm Springs - we have lots of open, desert space!).
Don't see why a person couldn't rig up a test bed using a mortar's powder chamber - built to safe spec., with lanyard firing - but without a lot of barrel length. Then safely affix different tube diameters/lengths and let the testing commence!
(I absolutely believe that safe, correct wads will be found to account for 25-40% of the "loud" that a tube can achieve)
Thumbs up to whoever said "NO" to Pyrodex.... BP! BP! And I also think the right granulation will account for a decent percent of "whump" from a tube too! (I didn't see a stickie on making BP...have to see about that)
my .02
wolff