The HMR is said to be about 50 or so yards more effective than the Mach II.
I've watched THR's Justin kill prairie dogs with his Mach II, out to 200 yards. Taylorce1 as a witness, calling the range with a laser rangefinder.
Justin was new to his rifle/cartridge, having been mostly a pistol shooter. It took a bit to figure the wind and the holdover "out there", but once he got it worked out, it was Dead Dawgs.