Indy,
D'Arcy Echols chambers the .300 Weatherby in his Legend rifles with a custom reamer with somewhat less freebore, just enough so factory ammo will work without excessive pressures. Of course, .300 Weatherby factory ammo is loaded right up there, but not as much as it used to be!
One of the reasons for the 6.5 Creedmoor is it's short enough to "chase the lands" by seating bullets out further as the throat wears.
I think recoil is a major factor in accuracy, the reason long-range target rounds have kept shrinking over the past few decades. By chance, the guy who holds the 1000-yard record for the smallest 10-shot benchrest group at 1000 yards, Jim Richards, lives right here, and is a member of the local rod & gun club, partly because our range goes out to 1000 yards. He shoots a 6mm Dasher, a cartridge quite a bit smaller than the 6.5 Creedmoor, using 105-grain Bergers.
Yeah, .338 magnums with heavier bullets do kick somewhat more than .300's, one reason a lot of .338 magnums used for long-range shooting with heavier bullets have muzzle brakes. I've only fooled around with a couple of .338 Lapuas, but each came with a brake, right from the maker!
My own present .300 Weatherby is one of the original South Gate sporters made on FN Mauser actions, the first "standardized" Weatherby rifles. With scope it weighs slightly over 8 pounds (just about the same as the 7mm Weatherby ULW I hunted with for a while) and I wouldn't want it much lighter, especially when shooting 200-grain loads!
