I couldn't get my 929 to shoot ANYTHING to within 6" at 30'. A trip to the mothership (for a forcing cone that looked like it was cut with a hatchet) didn't improve it either. Never did figure that one out but am very pleased with the fit,finish and accuracy of the 986 that replaced it.
My 929 is stamped 9MM on the barrel - not 9x19 or 9x21, just 9MM. It's my understanding that this was done to make importing it into jurisdictions that banned military cartridges possible.
I purchased mine last year and ordered 500 starline 9x21 cases. 95% of my shooting has been with the 9x21 cases and it's easier to keep track of them that way.
I have no idea if it could handle 50,000 PSI loads and there no way I'd test it in my gun.