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Getting Started: 6mm BR Norma neck bushing question

By knowing the specs on the chamber or reamer.

No way of telling what reamer spec was used if it's a factory rifle.

In general, Lapua brass runs 13 to 14 thousandths a side. So 26 to 28 thou for brass, plus 243 for the bullet gives you 269 to 271, and you subtract a bit for springback, yielding 267 to 269. I run a 267 for no-turn at the moment. I started with a 268, as I recall.

You'll likely eventually end up with a few bushings; I'd go ahead and get several centered around where you're likely to be. A little extra money now is better than having to wait for a new bushing to arrive to be able to shoot.
 
No way of telling what reamer spec was used if it's a factory rifle.

In general, Lapua brass runs 13 to 14 thousandths a side. So 26 to 28 thou for brass, plus 243 for the bullet gives you 269 to 271, and you subtract a bit for springback, yielding 267 to 269. I run a 267 for no-turn at the moment. I started with a 268, as I recall.

You'll likely eventually end up with a few bushings; I'd go ahead and get several centered around where you're likely to be. A little extra money now is better than having to wait for a new bushing to arrive to be able to shoot.
And….cerosafe is your friend. A call to the factory would narrow things down. When one deals with wild cats/non SAMMI cartridges can be a big learning curve.

I will admit to buying a few “pigs in a poke” myself.
 

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