N540 should be fine starting at minimums - like we always should. I doubt seriously that the BR would hold 4 more grains. The cases are just too close in spec.
SAAMI does not have the 6mm BR Norma so I've attached the C.I.P. (European) cartridge drawings for both the 6 ARC and the 6 BR Norma so you can see that apples to apples. It's in mm though. For example, the powder chamber of the BR is 0.75mm wider, but the 6 ARC powder chamber is 1.17mm taller. They are that close.
As for N540, if you read the link I posted you can see that these 500 series powders are similar to the Superformance powders used by Hornady. You might have missed that in their description....
"Vihtavuori N540 delivers outstanding accuracy with exceptionally clean burning. Due to the extra energy content that all of our N500 series powders include, a velocity up to 30-40 m/s higher can be achieved with the same pressure level compared to the equal in the N100 Vihtavuori series. It is certainly worth trying when using heavier bullets and when higher loading densities and muzzle velocities are needed!"
Finally, in the same N540 link I posted, all the way down the page they have all the cartridges that have been tested. They include the Valkyrie and the Grendel...
The 6 PPC is less comparable only because the loads offered are for lighter bullets only....
The main advantage of the 6 BR is Lapua factory ammo and brass.
The C.I.P. cartridge drawings are below, they include pressures as well.
Well, I happen to have both of those cartridges on hand, so I took some measurements (case, projectile, and cartridge length, plus actual fired case volume in gr of H2O) and plugged them into Gordon's Reloading Tool.
6mm BR, Lapua fired case:
Case length - 1.555"
Measured case Volume - 38.7 gr H2O
6mm ARC, Hornady fired case (once fired factory brass, fired in a Proof AR15 barrel):
Case length - 1.481"
Measured case volume - 34.4 gr H2O
Using my current lot of 108 ELD-M bullets, I get a measured projectile length of 1.262."
6mm BR:
Seating this bullet to where the top of the boat tail is even with the bottom of the case neck, I get:
Cartridge length - 2.380" (or 60.45mm, which is 1.55mm shorter than 62mm COAL from the CIP diagram you provided).
Effective case volume per GRT - 34.05 gr H2O
6mm ARC:
Using the CIP cartridge diagram you provided (which btw is 3 thou longer than the factory Hornady 108 ammo I have on hand) I get:
Cartridge length - 2.256"
Effective case volume per GRT - 29.23 gr H2O (4.82 gr less than the BR, or 14.16%)
Taking it a step further and asking GRT to run Vihtavuori's 27gr starting charge of N540 for the BR (notably for a 105gr Lapua Scenar, not a 108 ELD) in each model (specifying a 26" barrel), it gives the following outputs:
6mm BR (84.2% load ratio):
2577 fps with a max pressure of 42112 psi
6mm ARC (98.2% load ratio):
2739 fps with a max pressure of 57561 psi (still within safe pressure for a bolt gun, but well above the point of damaging a gasser)
Admittedly these are models, and each would need to be trued to a specific rifle to give the best possible estimates, but I think they illustrate the point I was trying to make about the difference between the two cartridges.
The BR starts with a 12% larger case capacity and is generally loaded longer (how much so depending on the bearing surface of a given bullet) when compared to CIP spec 6mm ARC ammunition. The 108 ELD is stuffed down into a 6 ARC case to the point where the boattail is sitting below the case shoulder. If the BR model I used was loaded out to CIP length the difference between the two would be even greater. Also, using a 6ARC case fired in a tight bolt gun chamber might slightly decrease fired case volume and further increase the difference in pressure between the two cartridges.
So what you plan to do with your bolt gun is probably fine, but BR starting loads are not apples-to-apples equivalents to starting loads for the 6ARC, even in a bolt gun.
Moreover, the majority of people loading for this cartridge are doing so in ARs. For them, the ARC and the BR are absolutely not apples-to-apples, and assuming such would likely result in someone using a "starting load" that is already above maximum "gas gun" pressure.
I did a lot of research on uppers/lowers/glass etc. And I ended up with these parts. I’m a pistol guy normally, so I’m just getting in to the rifle world.
