Boyd, this morning I measured a new Norma 6BR case and a new Lapua 6BR case for comparison. Both cases are recent production. Measurement 'A' is the very rear of the solid case head, right before the extractor groove starts. Measurement 'B' is .200 ahead of the rear edge of the extractor groove.
Norma 'A'-
.4670
Lapua 'A' -
.4695
Norma 'B'-
.4655
Lapua 'B' -
.4674
For guns with the original Remington spec. BR chambers, the Norma cases are a viable alternative to rechambering for the Lapua cases. Simply trim the neck on the Norma 6BR and you're in business. My suspicion is that the original poster might consider this approach to get ahead of the problem if his chamber proves a bit snug for the Lapua cases.
Here's two that I use the Norma 6BR case in.
The top one is a 22BR with the original Remington 'BR' spec. chamber (1.520 long). Norma 6BR cases are run through a Redding 22BR Body Die, trimmed to 1.500 and then neck turned for the .245 chamber neck. It's my go-to rig for fox and coyote hunting. No safe queen....home brewed camo paint, beat up, dinged up, scratched up, hunted hard and heavily bloodied...literally. Ted Nugent's signature and Spearchucker stick figure adds some good Karma and cred to it.
The bottom one is a 6BR 'dog gun with a Lapua spec. chamber and .265 chamber neck. When it was finished, there wasn't any Lapua brass to be had so I used the Norma to start with. I've tested the Lapua cases back to back with the Norma and if there's any accuracy difference, I can't see it. The 25 Norma cases I started with have been fired over 20 times with no loose primers or lost cases. Loads are 55 gr. Nosler BTip @ 3,700 so they get leaned on pretty hard. Ditto the Nugent connection on this one, as well.
For what it's worth. -Al