What is the 6mm BR?
The 6mm BR that is most commonly used today is also called 6mm Norma BR, "6BR Norma", or just plain "6BR". Norma started with the 6mm Remington Benchrest and increased the base dimension slightly. But the official SAMMI cartridge OAL is the same: 1.560". (However Remington did originally produce brass at 1.520" OAL and some older chambers have been cut to a shorter length.) Though Norma standardized the round, most people shoot Lapua brass, because of its superior quality and uniformity, not to mention lower cost. The case capacity of Lapua brass is about 38-39.5 grains of H20, after fire-forming. The exact capacity depends on your gun's chamber and the brass lot.