As fast as you can stand the recoil, which for most shooters is far less than what can be achieved by the rifle!! 300gr bullets in the 45-70 at ~2700fps has been achieved from the 32" BC, 2500fps from the 22". Considerably less for the .444 as it has 5gr less H2o case capacity and is loaded to 42kpsi where the 45-70 can be loaded to 50kpsi.
Tim