I was also going to suggest seeking the help of the range officer, if any. My general rule is if anything is goofy with a shot, stop and investigate throughly --which includes dropping the hammer on the round a couple of times.
I would have packed up the gun after that test and and if I was done shooting, bring it home. The fact that he left at that point may be coincidental. On getting it home, I would tap out the mainspring housing pin and remove the mainspring housing before going any further in my investigative disassembly or whatever.
Odd that both a misfire and an action lockup occured at the same time. Could it have been a failure of any firing pin blocks in the action which would have both blocked the slide and prevented the gun from firing?
Other possible causes: bad safety, where it was actually "on" but did not prevent the sear from releasing, bad loading with the bullet jammed in both the rifling and the case at the same time broken link, dirt in the locking grooves or under the link, both of which could have prevented a complete return to battery and an action lockup. In both these cases, the firing pin might not be in line with the primer's anvil.... just "blue-sky-ing" the problem.
Anyone got any other diagnoses?
Terry, 230RN