The GMX is a mono-metal bullet similar to the Barnes offerings. It is made using a slightly different metal that is a bit harder than the pure copper Barnes so it might leave less copper fouling in your barrel.

If you do choose a mono-metal bullet you will need a relatively high impact velocity to insure good expansion. You might wish to use a lighter mono-metal bullet (less than 140 grains) fired at higher velocity, or just stick to a time tested traditional bullet in a more normal weight of 140 grains.

I am trying some GMX bullets now. They shoot well at the range and leave an easily cleaned barrel, but I have yet to use them on game.