About four years ago, I killed a cow elk at 537 yards (ranged) with a 6MM-284. The 100 grain Sierra Pro Hunter, launched at 3430 fps, hit the cow broadside in the ribs and blew a broom stick size hole through both lungs. She ran about 50 yards and went down. Although the .243 runs about 350 fps slower, it should have about the same amount of "pop" at close to 400 yards. Bullet placement . . . .