The common 26" pencil thin barrels on the MK V rifles don't normally perform well with long shot strings. Attempting to define your Weatherby's worth by its group size is a little unfair as it was never intended to be a match rifle. These things are pure hunting rifles designed to fire only a round or two from a cold barrel to complete their mission. Once more than 3 shots are fired, you can expect the group to open up, hence Weatherby's 3 shot accuracy guarantee. So think of accuracy in a different way. Instead of a 3 or a 5 shot groups, fire two rounds at the target, and measure how far they are from your intended point of impact. Keep the target size realistic, and your .300 will perform admirably well beyond a quarter mile.
As an aside I recall a fellow shooting on the next bench to me when I lived down south and shot at a small range just north of Winnipeg. He was shooting a MK V in .300 Weatherby while I had a M-700 .30/06 topped with a cheap Bushnell 3-9X scope in Weaver rings. He had very nice equipment, an expensive bench rest, and a high end micrometer, the ones that look like a C clamp, for measuring his case head expansion, but he was driving himself nuts trying to keep 5 shot groups under an inch and a half and was getting frustrated in that many of his groups were closer to 2" than 1". I was happily banging away 165 gr Hornadys pushed along by 4831 equivalent bulk powder from Ammo Mart, shooting off a wobbly stack of sandbags and for the most part my groups hung around an inch to an inch and a quarter at the 100 yard line. The Weatherby guy finally threw up his hands and stormed off saying he couldn't shoot with the disturbance of my muzzle blast. Funny, his muzzle blast didn't effect my shooting. Anyway, his problem wasn't with his loads, his bench technique, or his equipment. His problem was simply that he had an unrealistic expectation from his rifle, an expectation by the way, he would never be put to use in the field.
