How can it be a 577/500 when the groove size is .463?
From what I see it's probably a 500/450 Number 1 Express 2 3/4" case. But I'd re-do the chamber cast and heat the barrels a tad warmer.. I think that's the problem.
It's ironic that there is another Turner listed on Gunbroker now. As nice as yours maybe.
-------------------- "When you see that in order to produce, you need to obtain permission from men who produce nothing; when you see that money is flowing to those who deal not in goods, but in favors; you may know that your society is doomed." Ayn Rand