Captchee
1) Twist. checked for pitting, minor at muzzle on one barrel, rest of the barrel is in good shape.
2) Barrels were rung. Sounds like a church bell. No buzz, no rattle. Just harmony.
3) Holes were retapped at 5/16-18, but the original holes were tapered. The top 1/8th inch the threads will not engage and it is hard to find nipples that are longer. Stainless 5/16 threaded rods were used, with a 3/64ths hole drilled longways through them (Not an easy task). The end was machined down to accept a nipple. Then I placed the nipple in a vice and put caps on it and fired it repeatedly, ensuring good discharge from the opposite end. I have not tested a back pressure scenario as of yet, but as long as the threads are good in both the gun and nipple I expect it will be OK, at least no less effective than a factory bronze nipple. There will be multiple remote firings before I shoulder it live, and lots of measurements before that, to check for wear. I also intend on affixing a modified nut for a shoulder on the nipple.
A test box has been designed that my father and I are building to contain the gun for remote firing. Plans are to mic the barrels at regular intervals and to check them after every shot as well as the nipple holes (we surveyors are anal about numbers).
In all likelihood, I would investigate building one in the future out of contemporary materials, but as of right now, my Dad (ex corp of engineers) is retired and likes to tinker, and I live by the quest. This is my current Holy Grail.