The impact of longer freebore (longer than needed not to jam the lands ) is HUGELY overrated. In some cases it can actually decrease the velocity you can achieve at a given pressure with a given powder. Some powders do not like low shot start pressures.
As a starting point, it's correct to assume additional freebore has NO affect whatsoever. The additional velocity of the Weatherby, to the extent that it exists at all, is almost entirely due to extra pressure. There is also some benefit to roughly an additional 6gr H20 of case capacity, but it's not much.
For heavy 175 bullets the Weatherby has the unfortunate feature that the slight bit of extra case capacity it has (~8% vs. RM and WSM depending on brass choice) causes it to want a powder slower than Retumbo, and there aren't any good options because the 50 BMG powders have received limited development and aren't very good. That's why instead of being say 100 ft/s faster like you might expect for the extra case capacity, in reality it's only 13 ft/s faster than the WSM at the same pressure.
Last edited by Llama_Bob; 04/13/19.
