SpudFiles

muzzlebrakes

there is plenty of debate on whether or not they make a firearm louder(as in decibels). the more efficient brakes channel exhaust gases rearward, this will provide a forward thrust of the firearm, thus reducing recoil. the redirected gases are what create the perceived increase in noise. depending on where you are standing the noise could be greater or lesser. behind the muzzle would be louder than in front of it.

somethings to keep in mind with muzzlebrakes. the smaller the bore of the rifle the more effective the brake will be. the larger the case/chamber capacity, the higher the muzzle pressure will be and the more effective the brake will work at decreasing felt recoil.

i could see a brake working better on a .75" pneumatic than a standard 1.5" combustion

If you want the cannon to be louder in the downrange direction, you could mount a rocket nozzle downstream of the burstdisk.

I don't know enough about nozzles to tell you how big to make it, but I do know that its construction can be as simple as an 11* cone. (I think that's per side, but I'm not sure)

For general loudness, I recommend an afterburner, or at least something like a thrust augmenter.
(ok, that idea is pulled out of my butt... but an after burner would be COOL!)

I LOVE my spud gun loud. It's half of what makes it fun. I say the louder the better. I live in an unincorporated area where people fire shotguns at 6am sunday morning within an earshot. no one calls the cops because the sherrifs dept can't do anything. The louder the better.

Have you ever seen a Dishka on a paintball guns barrel. Without any porting on the barrel (or plugged porting) the Dishka is a cone shaped device on the end of the barrel that amplifies the sound significantly.