Killshot said:
That bushing size is good for a .270" neck chamber.
Skipping a few steps, in general, that's going to be about right.
Just to be clear for our OP, who is starting in the game, the tension doesn't depend on the chamber neck diameter. It depends on the brass.
Sized Brass
with Bullet Seated - Sized Brass
without Bullet = Neck Tension.
I should have been clear about all that when I recommended a 266 bushing to the OP. I.e., my recommendation assumes a .270 or larger chamber neck and the blue box Lapua brass.
The backstory is that the older, gold box, Lapua brass had neck thickness such that a no-turn neck chamber was .272. Brass was around .270, so .268 gave .002 neck tension.
The new, blue box, brass is thinner in the neck so as to be more in line with the published chamber specs for the 6 BR. It's going to come out about 268. Thus, the 266 bushing recommendation.
NOTE: All that is subject to you verifying actual measurement. Postulatin' takes a back seat to Measurin'
My one whine for the morning - It might have been a great thing, long term, for Lapua to get their 6 BR brass in line with some spec some where. It sure was inconvenient for those of us with 272 necked barrels and 272 reamers.
Fortunately, I have bought up enough gold box 6 BR brass to wear out a couple of barrels.