Magnums are supersonic. They are tough on the brass, though, which makes reloading somewhat limited.
I was referring to the rate of pressure rise in the chamber, not the round velocity. Any sudden pressure above an Atmosphere can produce a shock wave, and there is a shock bottle at the moment of exit even with a .22RF.
When a pilot exceeds Mach 1 it is rough for a while while they penetrate the "wall" of the shock wave. Leakage between the cylinder and frame probably happens at the start and end of the pressure spike, so the actual volume of gas lost should be miniscule.
There are revolvers chambered for the .45ACP. It will be interesting to see, given identical barrel lengths, what the Chronometer says about muzzle velocity in an Auto vs. a Revolver! Has anyone seen such stats?