By the way you told it, I just know from messing with these S&W's since I was 17. I hope it is the ammo. When the lock fail's it is due to a bad spring most of the time (one on the flag) and a burr where it makes with the hammer. Anyway with preasure on the trigger it wouldn't loosen up for you to finish firing it, but carbon under the extractor or lead on the cylinder face will give with preasure. If it is so you will know right off the bat the next time. Most of the time I learn something new every day (don't tell the wife).