Question concerns regular assembly operations in Remington, Ruger, Winchester plants...

When most factories have an assembly operation, the assembly line operator reaches into the bin or conveyor belt and grabs the next available parts and assembles them together. There is no claim or expectation of trying to match tolerances among parts. And consumers and repairmen expect if a part needs to be replaced in the field that whatever replacement part sent from the factory or warranty station will fit and the repaired machine will function as well as the original machine.

Is there any expectation that a rifle or handgun factory is different ?

Does the Remington or other worker place the action in the proper fixture, grab the next barrel from the bin, screw the barrel in against the go/no-go gauge, and send it to the next step ? Or is there a custom fitting step ?

Hammer

I toured the Savage Plant, and was impressed with their procedures.

Barrels are chambered from the rear, and in a vertical position. They also then use a Flex Hone to polish the chambers.

All barrels are installed very carfully by an "old" man, and he hand checks and hand tightens the barrels to minimum headspace.

All guns are proof tested before shipping too.

Believe it or not, Pull through reamers are preffered by many when doing Military Matchgrade SLR's like the M1a.. The way they actually do it is in a machining center, when they thread the barrel(with a geometric die head BTW, not single point), they ream the chamber and set the shoulder position all relative to one another. It is very fast. Keep in mind that Remington reams at a pace a BR smith would consider obscene, and the reamers are sharpened over and over, sometimes by less than skilled people. When we chamber a gun, don't we inspect the reamer before starting for things like nicks and gouges? Remington might do 400 barrels a day, there simply isn't time to inspect the reamer each time.

Toolmaker

I seem to remember hearing that Remington (for example) simply screws a unchambered barrel onto the receiver, than sends it down the line. At another work station a "pull through" style reamer is used to cut the chamber. With the bolt pushing the reamer, headspace is set when the bolt handle finaly closes fully.
It's crude (and beats the crap out of the rifling) but it's cheap, fast, "close enough" and dosn't require any skilled labor (perfect process for a factory enviroment)

Interchange ability of parts was developed by the firearms industry, if not for the early work in the industry we would not have many of the machines we now take for granted.

It's a fascinating story, and well worth a bit of research.