Dont discount the 243 Win.
Wife got this guy at 547 yards. Horn 100 gn. SPBT. One shot went about 3 ft. Home made rifle w/Model 70 Action.
6.5 CM is hard to beat. 665 yards, 143 gr. Hornady. DRT
The real difference is not the gun/bullet, but the fact we spend all summer playing on BLM property 2 miles from the house where we can shoot 2000+ yards.