I've read that the gmx needs a minimum of 2000fps on impact to initiate expansion, compared to 1800fps for the Barnes tsx, and 1600 for the nosler e-tip. I guess gilding metal is that much harder than copper. Seems like it would just be a solid unless it was a fairly close shot with a hot load in 243. I guess with the extra velocity of the hornady superformance ammo it's less of an issue.
Nosler lists 1800 fps minimum for the E-Tip, which is made of gilding metal like the GMX.
