why some precision rifle builders do not use bushings
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.
why some precision rifle builders do not use bushings
Ok so I’ve been wondering about something. We do this extensive setup with our Interapid on our almost perfect Japanese bearing’d many thousand dollar tool room lathe using a barrel we have no way of really measuring bore and groove diameter to perfection on unless we have an air gauge. Axial and radial alignment are as perfect as we can get if we convince ourselves our 2.75” long $400 perfectly solidly mounted test indicator is really measuring half a tenth. We taper bore with our solid carbide boring bar just leaving enough to clean up with the reamer that we have no control of the dimensions of (unless you have an optical comparator, then we can send back and have the toolmaker try again, unless we also make the reamer). So, for all intents and purposes, we’re as perfect as we can get, start the pressurized oil system and go to work with our couple hundred dollar “floating” reamer holder we bought from JGS or Manson or Kiff or made ourself. We turn it around and cut a crown, either single point or with a piloted cutter with the perfectly sized bushing since we have every size there is for our particular bore diameter. Straight, recessed, 11 degree, however we like it, polished but sharp. Then we screw it on our perfect custom action we have no control over in our perfectly bedded perfectly stable stock. We put a scope on it we have no control over tolerances on in perfectly lapped rings. Then we go shoot it. It’s perfect.
Just one question after all that: How many rounds will go down the barrel before erosion takes out all of our careful work getting a perfect handoff from case to rifling and we get to start over? One match’s worth?