PA Bob, let us know how everything turns out. My M60 also has a bit of constriction where the barrel is screwed into the frame. I don't have pin gages to measure this, but can see it after cleaning the barrel up to a shine and looking through it with the naked eye.
Even with the tight throats and barrel constriction though, it shoots really well with commercial cast bullet, and jacketed hand loads. In fact, it will out shoot a 5.5" 45 convertible Ruger Blackhawk I've owned and tinkered with for several years now. (Which is another story and on going pain in my butt).
