DougGuy
Yep, although I still use lots of reamers for other jobs, reaming cylinder throats is a thing of the past for me. After investing in the Sunnen hone two years ago to finish cylinders after reaming, I have discovered a few things that are not readily apparent and this caused me to rethink the whole operation.
First and most importantly, let's look at the metallurgical properties of cylinders. Let's use the Ruger Single Action revolver as an example, since this is the most frequently encountered cylinder. Afaik, Ruger uses long rods they get from the steel mill on rail cars to make cylinders. These rods are already heat treated by the steel maker, and they are made of an alloy that is very tough, it is granular, and fairly inconsistent in hardness. I have reamed cylinders that had 3 throats cut normally, and 3 that nearly stripped the shank off the reamer trying to turn it. Same results with other cylinders having all 6 throats excessively difficult to ream, and others having only 1 throat out of 6 be very difficult, and the others ream with no difficulty.
That being said, the throats that are quite difficult to ream, finish considerably smaller than the throats that cut normally with the reamer. I have even forced a .4525" reamer through a throat, only to have a .452" pin gage absolutely refuse to go into the throat that JUST passed the .4525" reamer through, albeit forcefully. How is this possible? Is there this much springback in the steel the cylinder is made of? There is certainly some, does the reamer "crush" as it is forced through the throat? Quite possibly, although it is made of high quality tool steel and hardened. Seems unlikely but I have documented this phenomenon more than just a few times in my shop and I honestly do not have a logical explanation for it.
It is for this reason, that it is physically impossible for a reamer to cut consistent sized throats in a cylinder where the hardness or heat treatment varies from throat to throat. You simply CANNOT achieve consistent throat diameters using the reamer as the only means of addressing the throat. You have to arrive at consistency by "finishing" the operation with an abrasive of some sort that can gradually enlarge the throat to the desired dimension while cleaning up tool marks left by the reamer.
Now I totally rely on the Sunnen hone to enlarge cylinder throats, it is slower but it is more precise and it will hone an egged hole round, it will hone a belled throat parallel, it is easy to "feel" the throat as the hone turns so I know where it is tighter and where to let the hone ride a little longer to even out the throat.
Some S&W cylinders have a weirdness where the chamfer area just ahead of the chamber is considerably harder than the rest of the throat, I can hone them and a pin gage will go into the throat 3/4 of the way but not all the way, and I have to ride the back of the throat on the stones to prevent belling the throat being the metal in front of the chamfer is softer and hones much quicker. You can feel this with the gage pins and you can feel it with the hone as it runs. Just an oddity I thought I would mention.
The Ruger cylinder where half of it cuts easy with the reamer and half cuts impossibly hard? Those harder throats take 3 and 4 times longer to hone them to final diameter than the softer throats on the other side of the same cylinder. You go slowly and start checking with smaller pin gages, once I arrive at a half thousandth under, I use that pin to gage the drag in the throats and at this point I want to even them out, hone a little bit more on any that are a little snug, until all the throats feel the same, then I take them out to the finished diameter and use the appropriate gage pin for the caliber to assure all the throats are the same size and the same amount of drag against the gage pin. This tells me that throats are usually within .0002" of each other, and that the customer will be very satisfied with the improvements and the gun will respond in like manner once boolits are sized to a light drag fit in the throats.
THIS ^^^^ level of consistency cannot be achieved with a reamer by itself!!!
I have two rather expensive dial bore gauges that I used in checking my work and I found out that I can feel with the drag against the gage pin the differences in throats that are more than a couple of tenths of a thousandth variation from each other. If there is the same amount of drag on the pin, or a boolit used as a gage, it is closer than any scientific equipment such as a Ransom Rest will be able to differentiate, much less firing from conventional positions or a rest.
In closing, the description given of Ruger cylinders is not meant as a criticism at all, in fact those cylinders are TOUGH, STRONG, and very well made! The inconsistencies in the hardness of the metal in no way compromises the safety or strength of those cylinders. If it did, Ruger would use a different process. The product they ship will withstand well in excess of +P pressures and so there is no inherrent danger in these cylinders as shipped from the factory and also no danger in having these cylinders "dimensionally corrected" for use with cast boolits.
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Originally Posted by Nueces
USAF RET 1971-95
