I need some peer review and confirmation here folks. . .
I wanted to apply my own scissors to cut through some of the mysticism surrounding cast bullets in Glock barrels. Today, I did some pin-gauging and bore slugging of several 9mm's. These included a new Gen 5 17 with the "Marksman" rifling, a Gen 2 19 with the polygonal bore, a Ruger P85, and a Springfield 1911-A1.
Chamber dimensions of the Ruger were a skosh larger (presumably deliberately sloppy for military applications), but both Glocks and the Springfield were exactly what you'd expect to chamber the cartridge blueprint shown in my Lyman manual - about .391" at the chamber entry tapering to about .382" at the front. Nothing unusual there
Now as for the bore:
It looks like both Glocks from roughly a quarter-century apart have tapered bores.
The Gen 2 19 will take a .346" pin gauge from the back, but it will only allow a .345" from the muzzle. I had a record of a previous slugging of this bore that gave a groove of .356" (don't recall if that was a chamber or full push through)
The Gen 5 17 is similarly .347" and .346". This gun also got a slug started into the chamber and then backed out, which measured .356", and a second slug pushed all the way through the muzzle came out at .355".
The other two guns were both .346" bore and .356" groove.
Seems that a .357" bullet solves the problem for all of the above.
It seems to me that if folks are basing their sizing diameters off of slugs that are pushed completely through these tapered bores, they will be reading the narrower muzzle diameter, and end up sizing possibly 0.001" smaller than properly desirable. Which might explain the leading horror stories that seem to circulate.
Such is my thinking. As I say, peer review and confirmation would be appreciated. Also curious if this tapered bore theory tracks across the other Glock calibers.