I've always been facinated by these and finally broke down and went thru the minor hassle of acquiring one. I'm glad I did, talk about fun to shoot!
I've learned a bit in the process, 115gr bullets will not run in this, I believe it has something to do with the recoil impulse due to back pressure. Whatever the cause, the result is when the chambered round fires it ejects the empty. Also the first round on top of the magazine, then tries (and fails) to chamber the second round down.
Switching to 124 or especially 147gr bullets cures that. Cycles fine and using the 147gr's it is very quiet. Somewhere around a .22, but if you stand back around 10 yards or more, it is very quiet indeed. Just a muffled 'pop'.
It took 3 months to get the paperwork thru, the first NFA weapon I bought took 6 months to the day, hopefully the next will run in a reasonable time frame.
Sig Saur P226 TAC, with YHM 9mm Cobra. A combination I'm growing to love very much.
RD