As has been stated, weatherby chamberlngs have a lot of freebore. From what I've heard, boat tail bullets can be more sensitive to freebore (and any defects the muzzle crown might exhibit) than flat based bullets. Simpler bullet designs are often more accurate, so I'd try a conventional cup and core flat base bullet (SSA suggested this too). It's my experience that in finicky rifles bullet to bore bearing surface is your friend (again flat base soft points are easier to work with than boat tail vld type bullets). Additionally, in my (very limited) experience, cartridges with large case to bore ratios (such as your weatherby-I have a win mag) generally shoot much more accurately with heavy for caliber slugs. A case full of powder, 100 percent load density give or take a few, is generally beneficial with such large cases. I have not noticed any difference between match primers or plain primers. Hornady interlock, Speer hot cor, Sierra pro hunter. 180 grains minimum. Slow burning powder 7828 at the fastest, fill right up (find a powder that allows you to do this with safe pressure of course, I've heard good things about h1000 and retumbo, have had good experiences with imr 7828, 7977, and rl25 in my .300 win mag. Perhaps look into getting the muzzle professionally recrowned by a gunsmith.