It looks like we're talking about at most an .002" variance? That's holding a pretty good tolerance. Way to go Ruger.
That's one school of thought, but it's not held by cast bullet shooters. In fact, accuracy-minded jacketed bullet shooters wouldn't be too happy with it. Making 9mm Luger sized throats on my .357 magnum is not what I consider "holding a pretty good tolerance." .358" or .359" throats would be more appropriate to caliber. If the throats are being cut much less than .357", it's because they're using an old, worn, undersized reamer to cut them. It wasn't possible to chamber some loads in my Blackhawk. Ruger refused to fix just the cylinder, they wanted me to pay to ship the whole revolver back.
Unlike Bill, I don't mind slightly oversized chamber throats, as this doesn't harm cast bullet performance nearly as much as undersize throats, providing you size your bullets to match the throats. No big deal, and no money wasted having undersized throats reamed out to the proper size. Jacketed bullet performance is usually acceptable, even when throats are .003" oversize.
I also think that pretty much everyone
can agree on the "correct" size for chamber throats when the barrel measures .357" across the grooves. The correct size is no less than .357", and no more than .359" in a mass produced gun.
Just my opinions, and worth what you paid to read them.
