The threads are produced at the factory with a tap and are often out of line with the center of the action and may be crooked as well. A piloted tap will correct this to a large degree ( I believe Guntech does this) as will single pointing the threads. The result will an action where the action, bolt, chamber and bore will all be in line maximizing the chance for an accurate rifle assuming of course the barrel is threaded and chambered correctly. Once this is done factory barrels can no longer be used as a custom diameter barrel tenon needs to be cut for action.
Where on earth did you come up with this one????
Bushplane, this guy is either pulling your leg or has absolutely no idea of what he is typing about and just making it up from some very rudimentary shop practice skills he picked up in junior high school.
Just ten years ago, many receivers and barrel tenons were cut on a lathe and chambered on a lathe. This usually took at least three separate set ups and operations. Someplace along the way, the separate set ups may have been less than perfect. This was usually the cause of off true threading and facing.
These days, where everything is computerized and as much as possible is turned out on CNC machinery, non concentric actions/barrels are almost a thing of the past. A skilled gunsmith is pretty hard pressed to better an off the shelf production rifle built on one of the new fangled, computer directed machines. Most of these rifles will shoot factory ammo into sub one inch groups consistently. Especially if it is equipped with good parallax free glass and the shooter knows their stuff behind the trigger.
A good gunsmith, will have a special mandrel that has been threaded for the type of receiver he is truing. Each type, needs a different mandrel.
See if you can Google John Hinnant's book of rebarreling. There is a complete shop essay on which jigs, mandrels and other tools that are necessary to properly true up an action and how to check a barrel for being threaded in a concentric manner to the bore/chamber/tenon face.
Since the very first firearm was produced, where the barrel was attached to a receiver, lathes have been used to do the work. I won't say it can't be done by hand for the plain simple reason that original threads were often cut with files. I have also seen some incredibly handy and skilled/patient people do things I wouldn't have believed possible.
Personally, even though I am very good with hand tools, I wouldn't attempt it.
I have seen some very decent chambers cut by hand with pull through reamers and even with speed wrenches. I have never attempted to do a complete from scratch job my self. I have finished off pre chambered barrels by hand, with a speed wrench but that was only a few thousandths of an inch and I sweated profusely every time. You need to exercise extreme caution to do it right.
My lathes will not run slow enough IMHO to thread barrel tenons. I have a small motor, attached to a gear box that I can bolt onto the base frame my lathe sits on. I also have a corresponding sheave attached to the back of my four jaw chuck so that I can drive the lathe with this system. It's awkward but it works well and turns the lathe around 20 rpm. It doesn't make for deep cuts but that's OK. Time isn't an issue when you're retired.
If you're really into changing out your own barrels on a regular basis, may I suggest purchasing a Savage??? Fugly I know but they are good rifles and shoot well. The barrels are locked in place with a universal lock nut that allows the headspace to be adjusted without needing a lathe. You will need a special wrench for the nut and a proper barrel vise to hold it securely while you loosen off the nut.
I can't make it much simpler than this. To explain it all, would take up way to much space and time. Now, Google or your favorite search engine are your friend.
