I finally got around to doing a Cerrosafe chamber cast on my M1888 Trapdoor Springfield. Not having a proper v-anvil micrometer to check the 3-groove slug I though I would measure the diameter of the throat where the rifling grooves "petered out." HA! There ain't no throat. The rifling grooves run right into the angle at the end of the chamber, the grooves farther back than the lands of course.
I tried two methods of measuring the groove diameter. I wrapped a piece of .0085" thick target paper around the slug and measured (at the corners of the lands so there would be little or no deflection) and got .479". Subtracting 2x .0085" I come up with .462". Checking across the corners of the naked lands I also got .462".
The chamber diameter at the case mouth was .482". My Winchester brass is .010" thick. Adding it all up a .462" bullet will just fit.
Now I just need a .462" bullet....
And no I 'm not going to turn case necks. I can't see the sights well enough to justify that.