Given the fact that I'm here, you're there, and your Gunsmith has discussed this with you, I don't think I'd push it one inch... Find something a little more tolerant of muscle. Just my opinion..... Good Shootin!
My 'smith is telling me that the cone breech on the pre-'64 Winchester is a potential weak point for "hot-rod" cartridges; he says that I'm flirting with case-head seperation if I try to jack the 6.5 WSM that I'm building.
Don't get me wrong; I'm not really a hot-rodder to begin with, but I'm wondering how careful I might have to be when I'm working up a load; I'd hate to blow up a brand-new rifle, not to mention all that bad stuff that might happen to me when I'm holding the thing.
I've always been of the opinion that Ruger #1's and Freedom Arms revolvers are the best test-beds to try and push envelopes; if you can blow up either one of those you're REALLY out there somewhere.
I just hadn't ever thought much about the 70 and the coned breech; obviously it can't be TOO terribly bad, or Model 70's would be blowing up all over the place, but I thought maybe someone would know a little more about it than I do.