The Federal Castcore load is a lower intensity, lower velocity round than are the Buffalo Bore heavy 41 mag loads. Neither in limited amounts should do any harm to your Mountain Gun, but recoil, especially with the BB load, could be substantial. I also have a 41 mag Mountain Gun, but have yet to fire it. I have bought some of the Federal 250 grain Castcores with the intention to use them in it at times, along with several of the standard 210 grain loads.