My understanding is that one big reason to go through the headstock is so that you can ensure that not only the mouth of the bore is concentric (to the chamber you are about to cut) but also that where the throat is going to be once the chamber is cut is concentric. Since the rifled bore can and does wander when they are drilling it that’s not a given. So, you may end up with the muzzle end of the barrel running “true” at the outboard spider or you may not; that’s not the goal. The goal is that the new chamber be running true to the bore at the chamber end of things.
To that end you could use gauge pins in the bore at the chamber end, or you can use a long-reach indicator to reach in and directly read the bore where the throat will be. That’s what I do. You are reading the lands/grooves so it’s kind of a pain. It’s a spendy little gauge. I’ll take a pic of it when I head back out there today.
This is why you use a spider chuck with rounded “jaws” (screws) rather than a 4-jaw: so the barrel can pivot on those round ends when you adjust at the outboard end. With the 4-jaw and its square jaws you could index in the bore (at the soon to be chamber mouth) there, but in terms of tilting the barrel to compensate for a wandering bore, such that the throat area ended up concentric, no. You’d be locked into whatever axial alignment the chuck jaws created.
It’s a very finicky setup and takes me a long time.
The headstock on my Webb is quite deep. I made extender sleeves that slip over the muzzle end of the barrel and lock on with brass screws. Works great.
At least some of what I typed above is mooted by at least some of the available floating reamer holders. You’d think a piloted reamer would also moot it, but things flex way more than folks usually imagine…
Maybe this will help visualize… I have a 7 SAUM donor rifle I bought recently and the plan is to make a 6.5 SAUM light rifle (not sure how light yet). I’m about to go on a 2-month wilderness backpacking trip (!) and for $$ reasons this isn’t gonna be the year for a new build. It occurred to me that I could rechamber the donor to 7 WSM, for which I have the reamer/gauges, and who knows, it’s at least possible it shoots great and then I have a toy to abuse for a while. I have mountains of 7 WSM brass, correct powders, etc. But here’s the catch. You know Big Green ain’t setting things up as above; so it’s possible, likely even, that the 7 SAUM chamber is not particularly concentric to the bore at the throat area. My new chamber is gonna follow that old one; the old one is the mutha of all pilot holes… and so my new chamber would end up also not concentric at the throat area. Course even given all THAT…… it might still shoot just fine.
