So, I'm a dumbass.
In my previous post, I put chamber length as 0.628. That was one of the outside diameters of the barrel, according to the GM schematic.
If a 9mm cartridge should extend 0.135" past the end of the chamber mouth, wouldn't that make the chamber length 0.750" - 0.135" = 0.615"? I might not have got a very good measurement when I measured it last time.