precision rifles, not hunting not tactical, benchrest type precision
we have to start with a basic understanding that rifle bores are NOT straight.
they "worm" down the bbl.
we are not talking big numbers worst case .003/.004 some less.
if you put a bbl thru the head stock or on rollers and turn it while looking down the bore,
you can see that the bore really is not straight as a laser( but good enough for a rifle).
this ain't new . trying to get a "straight" hole in a 30" blank aint easy.
the ideal condition in a chamber is to have the bullet engage the rifling straight. as in .0000 straight.most precision rifle makers use a .0001 indicator.
( some use rods with indicators, i do not use that method, but it works)
so the setup is to dial in the bbl in the lathe with the indicator at the intended THROAT.
this is why pre-bore, taper bore and ream without a bushing .
the slight worming of the bore can cause stress on the bushing, the reamer is steel and the back end of the reamer with move out of line causing and over sized chamber at the back plus if the bushing is allowed to follow the bore it can be a couple thou off from center when finished. look at the lands are the all cut uniform in length from start to finish the same start the same finish.
i had a worst case in a remington bdl i bought new.
the land on one side was only partially cut in the throat, on the opposite side the land was cut past the groove and into the bbl!
brand new rifle with the throat cut maybe 2-4 thou off center.

ask away
i will try to answer questions of those that are interested.