Update...
Got a new Spring Frame/Set, a new piston, and some other stuff.
First I checked the dimensions of the new piston... it is almost identical to my current piston in dimensions... maybe a few ten-thousandths, but nothing substantial compared to cylinder differences.
First test, I put my weakest set of old recoil springs back into the gun, but this time including the inner springs... again, the new barrel performed excellent for 14 shots. Enough for me to say "is working".
Next test, I put the new piston and new springs into the gun and loaded up 350gr this time. Wouldn't load these... it either failed to pick up the round completely, or grabbed the round between the rim and the case body for a stovepipe. I dropped my original barrel back on and it fired perfectly for another magazine of 350gr rounds.
I switched back to Hornady 300gr (because that is what was in the next two magazines) and the new .50AE Barrel. It was able to load the next round some of the time, even twice in a row once... but mostly fails to travel back far enough to pick up the next round.
As usual, my original barrel shoots every round without prejudice... the new barrel functions with my oldest, weakest springs and Hornady 300s.
Not sure what to think, but I am going to keep with it until I can figure out what is wrong, and how it might be resolved.
MrBlackCat