Pretty good test between the Barnes and the GMX here:
http://www.soarraptors.org/Documents/CopperBulletStudy.pdfNot much difference between them...according to this test.
The only thing I have shot with them is one antelope buck. It was a headshot so not much of a test of the bullet (165 grain .308 from a 30-06). In my rifle they are not as accurate as the Interlocks, but shoot around an inch and a quarter, at least in Superformance guise. Haven't played with them handloading yet, but hope to soon.