Its a copper alloy with the Hornady flex tip By combining the technologies of the GMX® and FTX® bullet design, the MonoFlex® bullet gives lever gun shooters another option. Constructed of a copper alloy, MonoFlex® bullets offer hunters a solid that won't separate, and when recovered, retains 95% of its original weight. Copper alloy differs from solid copper in that it is harder, tougher and does not foul or increase pressure the way solid copper bullets do.
Upon impact, the patented Flex Tip® design initiates immediate expansion, even at the lower velocities often encountered in mid to long range lever gun shooting situations. Its deep penetration and high weight retention combine to produce yet another accurate, deadly and dependable choice for lever gun hunters

Vincent, it is simply a flexible rubber tip that is soft enough to use pointed bullet profiles in a lever rifle tube and not set off the round in front as the tip sets right on the primer of the round in front. Cheers!

Vincent, MonoFlex just refers to the one piece (mono) soft tip (flex). One piece soft tip. MonoFlex

What He said... hahahha