Im once again thinking about making a 6br. This is supposed to be used for varminting. But, I want to use a barrel that is as short as possible. Reading on the webpage, about using the 6br as a varminter, they are claiming 3800fps with 55 grain bullets, and 3300fps with a 75 grain bullet. Since those are the primary bullets I want to use, how short can I keep the barrel and still get these velocities?