I've read some threads on other forums regarding this subject. Often cited is the method used by Steyr/Austria to manufacture Mauser pattern 98 military rifles. Also suggested for Yugo M48's.

Steyr military barrels and receivers are stamped on the undersides with +/- numbers from 0-5. Original guns will have receivers and barrels with opposing +/- numbers. So, a +2 barrel would be mounted on a -2 receiver.

Speculation was that barrels and receivers were manufactured and gauged for headspace dimensions. When gauged, they were stamped accordingly within a given variance range, +/- from a datum of 0 and given a deep index mark. The barrels would have sights pre-installed. Then, at assembly, corresponding +/- units would be mated and proper sight alignment and headspace achieved.

My question is: If an indexing method was used, was this achieved through standard lathe methods or was a tool akin to a Rigid pipe threader used? Insights appreciated.

Bob

My understanding is that the threads were milled, not turned or tapped.

None of the documentation I have seen has the sights pre-installed. To date no Steyr or FN documentation has been published, and none has been available privately. Not that it doesn't exist, I don't know about it.

Barrel and receiver threads were milled.