There is a lot of myths out there on the difference on these two chamberings, some say that the base dimentioned are different, and all sort of houpla. There is no diference in the brass. The diference was done because when the cartridge first came out it was chambered for 40x silhouette pistols. The original chambering was for light bullet, in a slow twist chamber. When the round was excepted as a rifle cartridge, using heavier bullets, and faster twist, Norma made the neck length longer on the case so that it would not chamber in the Remington 40x pistols. So the original Remington br case had a shorter neck, and that is the only diference. Most dies you will find are marked "6 BR Remington." but they will also work with the 6 BR Norma chambering.
The 22 BR is an interesting cartridge, it can be loaded down to 223 velosity, or up to almost 22/250 velosity. The BR case with its shoulder angle is fairly friendly on barrels. Barrel life will mostly depend on who much powder you burn, and how hard you push your ammo.
As far as nitriding any barrel, I have yet to hear where this was an advantage, and I have heard more horror stories about this than anything elses. I would recamend you not doing this.
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