Thanks for the pass fellas. I'm still puzzled how the loose threads got by me. The barrel cinched up tight when installed. It wasn't obvious until the barrels were removed... and I had done a lot of sundry work already oweing to my confidence in the barrel joint. In retro spec the evening I was cutting the threads my youngest son and a buddy dropped by to pick up a couple of his guns. I stopped work and dealt with them, shot the bull for a while then finished the job while they watched and the conversation continued. I'm guessing the distraction was a contributing factor. Also when re-cutting the shoulders yesterday - my first threading job since this one - the bit was quite dull, needing sharpening before the redo could be tackled. Possibly another contributing factor...
This wasn't a catastrophic event (not yet at least!) just noticable movement, which luckily showed up during the test firing process. Maybe 0.050" enough excess headspace to prevent firing although I don't think this had anything to do with the original subject of this thread. At any rate it sure emphisizes the need to proof every project even where its not a legal requirement! Mistakes DO HAPPEN regardless of the care taken.