SpudFiles

At 550 psi shooting a 10 gauge 44 gr plastic wad the Prochrono measured 1146 fps.

I heard a cracking sound that I never heard prior to that shot.

I believe that the plain wad broke the SOS based on the 'crack' and the reading.

I will try to reproduce it.

That is really cool. I need to build a high pressure gun...

When I read the title I was expecting something completely different

How close was the chrony to the muzzle? The blast can sometimes give misleading results. Not saying you didn't go supersonic but it's good to be confident in your instruments

I heard a similar crack when I fired a very thin nylon projectile from my 400 psi co-ax. The walls were less than a mm thick and the projectile was very light. The crack was definite and completely different to the muzzle blast and projectile sounds I'd heard from the same gun.

Interestingly, the projectile was made fairly stable. However it missed my backstop completely, which should have been an easy shot. Transonic/supersonic flight no doubt messed with the efficiency of it at those speeds.