I realize this is an old post, but I managed a half inch group from my 18" GLFA barrel shooting 300 grain XTPs. The Hornady black were a consistent 1.5".
I also average 80-100 fps over what's stated on the Hornady Black box as well as the Hornady reloading manual. Let me know if you have any questions. The guys at GLFA are super helpful, I ordered directly thru them.