The 250 gr FTX was actually designed for the 450 Bushmaster, This is a cartridge gun, a rifle which has a muzzle velocity of 2200 to 2300 FPS. This should be taken into consideration when using them in a muzzle loader. I use them with very heavy loads of Blackhorn 209 and a muzzle velocity similar to what they are designed for and the are very accurate and work well for me. I have heard complaints of them not opening well from those that shoot weak loads.