I should have been more specific. Cyls are not heat treated after reaming for a different cartridge. Cyls are not annealed for rechambering, the reaming does not affect the factory heat treatment, and therefore re-heat treating would only degrade the original factory heat treatment quality. Heat treatment doesn't have a depth of hardness, it's the same all the way through; it's not to be confused with case hardening. Rechambering reamers are sufficiently hardened and durable to be able to ream heat treated cyls.
It is my understanding that Ruger gets long rods from the steel mill via rail car, this is what they machine cylinders out of. It is heat treated at the factory, but I must truthfully say, it is VERY inconsistent! Hardness varies greatly, and there seems to be absolutely no rhyme or reason to this anomaly. I have been reaming and now honing cylinders for years, probably the 44 and 45 cylinders I have done are in the thousands.
It is VERY common to start reaming a 45 Colt cylinder with a Manson reamer, and have half the throats, the throats on one side of the cylinder ream normally, and the other half of the throats you think you are going to literally twist the shank of the reamer right off. The reamer will be extremely difficult to turn, it will often SQUAWK loudly, and I have had to beat the reamer through to finish the throat 3-4 times.
The end result of this unfortunate learning is that the throats in the harder side of the cylinder are smaller in diameter than the throats on the other side of the same cylinder that reamed in the normal way. I have tried to pass a .452" gage pin through the throat that I just beat the .4525" reamer through, and it wouldn't even start. Go figure!
Quite mystifying if you ask me! Reamers made of HSS will actually crush, if you look the flutes are not perpendicular to the center, and the cylinder, if it is hard enough, and you force the reamer, it WILL go through the hole, but it won't be the size you think it should be.
Some fellas get lucky, they ream their own cylinders and have 3 or 4 Blackhawks in 45 Colt that the reaming operation goes smoothly. And then there are the ones that buy the reamer and the pilots and the first couple of throats they try to ream turns into a wakeup call from hell when the brand new reamer won't cut the cylinder like it should. Welcome to Ruger's heat treat fellas, it is all over the map.
For this exact reason I gave up reaming cylinder throats and invested in a Sunnen precision automotive hone. It is stepless, there is no fixed size, you adjust it to hone to whatever diameter you choose, keep checking with gage pins, etc, until the final diameter pin goes through the throat.
Do I notice the same varying hardness with the Sunnen? Of course. It takes a BUNCH longer to hone those hard throats, but the end result is that they can be honed within a couple of tenths of a thousandth of each other, this right here, is the goal, consistent throat diameters
Having consistent throat diameters is the single most important thing about any revolver cylinder.
This CANNOT be done with a reamer, UNLESS you just luck out and that cylinder doesn't have any significant variation in hardness from one throat to another. I would say these even tempered cylinders would account for roughly 65% of the total production in 45 caliber, while the other 35% have hard spots and throats won't come out the same size when using a reamer.
Let me also add this.. Ruger used to use a Hitachi machine with 3 chucks and they would gang ream 3 holes, index over one hole and plunge the other 3. As reamers wore, they would replace the worn ones and if the other two were still serviceable, they would keep using them until one wears enough that it needs replacing, so it was very common that a cylinder would get reamed with 3 different sized reamers, each reamer would cut a "pair" of throats, adjacent to each other, and you would have a cylinder with 3 pairs of throats of varying diameter. It was the norm to find a 70s or 80s Super Blackhawk with a pair of .429" throats, a pair of .430" throats and a pair of .4325" throats because that reamer had been just recently replaced, it was new, and cut larger holes than the other two worn reamers.
You can also figure in the variations in hardness of the cylinder blanks as described above, and this would also attribute to varying throat diameters. Throats in the normalized steel of the blank are larger than the throats in the harder steel, all in the same cylinder.
Bottom line? At least they didn't punch them out one size fits all, they have been notorious about having tight cylinder throats, but this leaves metal in the throat that we can custom tailor to fit our needs. If Ruger did this? People wouldn't complain about cylinder inconsistencies, but then the price of the guns would be double.