Headspace is something you have to establish during fitting if you're putting up the bbl yourself.
Or you should check the specs of if it's a rifle already assembled and you're just the new owner and doing some extra work to it.
Either way make sure it's right and/or make it right before getting into the pretty work.
That's the idea behind the test firing/function firing while still in the white with a new custom build. Get all the kinks out first if there are any. You may have to do some minor adj to things, disassembly, reassembly,ect. It's just easier to do while in the white.
It's nice to have the rifle or shotgun look all nice a pretty, but if it doesn't work right it isn't worth a damn. Taking it back a part repeatedly while in the finished condition to tweak trigger mechanisms, bedding, feeding and any manner of things is a pain.
..and you shouldn't need a 4 ft cheater bar to put a bbl onto an action. Most Military rifles are put on with great torque and it seems that is the standard by which many go by. But it really isn't needed. A decent fit on those large dia threads ,a nice square shoulder and face,, all that simply tightened down with the wrench is sufficient.
Hot bluing,, it's a very good idea to remove the bbl as the bluing salts gets into the tiny voids in between the threads. Then the water evaporates leaving the salt itself in there and you get that white 'bloom' growing out of the joint.
It's just the salt absorbing water from the air. It'll cause rust of course but also destroy any bluing it grows onto.
Oil won't kill it. Plain hot water will disolve it (that's what the after- bluing rinse should have done) but it can't effectively get down into the threads to get everything out.
With rust blue it's the oil or grease that's already on the threads,,especially grease applied to the threads when assembled, that liquifies duing the heat of boiling and seeps back out over the rust blue coatings to spoil them.
Usually shows up as unblued metal areas right at the bbl to recv'r joint.
Sometimes you get it where bbl band swivels and/or front band sights are installed too.Depends how they were done.
You'll see it around a screw 'plug' repair often too as there a tiny amt of oil down in the threads of the plug. The oil weeps to the surface ruining the bluing in a perfect circle to reveal what was an almost invisable repair.
Sometimes it'll appear to blue OK, but will rub right off when you're done and are coating the piece with oil afterwards.
It's just that the area is contaminated and the blue won't adhere well.
It can also get onto the carding wheel if you card over those spots and pick it up.
It doesn't take much to contaminate the carding wheel and then you're laying it on allover the piece.
It contaminates the tank water each time too w/an oil slick on top.
