4mesh, I'll try to shed some light on materials stress and drilling straight holes. First off, I am not a barrel maker nor have I ever been in a barrel makers shop, but I did spend over 40 years in the largest job shop in Tennessee. Your experiment with turning Chrome-Moly (SAE 4140, 4150, etc.) (CM), your turning method induced most of that stress. Had you used coolant and positive rake tooling things would have come out much different.
When making gun barrels and drilling that long deep hole, the material, CM or SS, must be stress free. The barrel makers use heat treatment and sub-zero cooling (a cryogenic process) to remove stresses depending on the material they are machining. At the best they can do, there will be some curve in the hole. Mechanics of the metal removal process, like when the chips block the coolant flow, dulling and edge flaking of the carbide tipped drill, inclusions in the bar stock, and other factors, prevent drilling a perfectly straight hole.
In the late 1960's we had a job in the shop to drill a 1/8" diameter hole through the center of 1,800 pieces of CM that were about 25" long. This hole was for compressed air that operated a mechanical release of the yarn bobbin tube that this shaft rotated. We were successful in drilling an exceptionally straight hole even though functionality of this machine part did not require it.
We came up with a method to drill this very small, deep hole, not as the goal of making the hole straight, but as the goal of getting the gun drill operating at the required surface footage.
"Gun" drill is a term used by a special, straight fluted, carbide tipped, tool that was developed to drill gun barrels. Makes sense. However, many other uses of this special tool occur out in the world of machining outside the rifle/pistol barrel making process. These tools are made by tool builders like Eldorado and others.
http://www.dmetool.com/ (look at this site to better understand the physical design and operating requirements.)
Getting back to the story of the job we had. With the machinery we had available and the time constraint of the job, we did not have time to build a special machine or send the job outside. (In those days, we did it in-house, or else). So, the first problem to overcome was getting the drill bit up to the required RPM. We were planning on using an engine lathe spindle to power the drill but the lathes would not run that fast. To overcome the problem, we decided to spin the workpiece in one direction while spinning the drill in the other direction. As I remember we needed about 5,000 RPM to stabilize the drill and get the required Surface Footage (SFM).
We set two Monarch engine lathes tail to tail and lined them up with an elaborate setup of surveyor transits and an optical telescope. These lathes were in the 25 X 96 class (25" swing and 96" center distance). On one lathe the part was rotated, with one end chucked and the outboard end in a roller steadyrest rotating at 2,500 RPM. Just down the lathe ways sat the gun drill propelled, by the spindle of that lathe, again, rotating at 2,500 RPM giving us a total work-material to cutting-material relationship of 5,000 RPM.
(We used splined shafts and intermediate steadyrests,mounted on the lathe carriages, to feed the drill and the workpiece toward each other. A Rube Goldberg setup for sure)
The result of spinning the workpiece and spinning the drill, these holes came out exceptionally straight. I wonder why the barrel manufacturers don't do this....or do they??
(Please forgiving this rambling post. I'm in a rambling mood this morning. How does this help answer the original question? Hopefully it will help understand why barrel bores are not perfectly straight....none of them.)
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