I respect your love of the weatherby family, but as far as tight groups, the shoulder design of the weatherby case limits its accuracy compared to others, and as far as handloads,maybe you dont want to do it, but thats how you bring out the accuracy in a rifle.

Try the 300 Ultra mag. It falls somewhere between the 300 WBY and the 30-378 WBY, as far as power and velocity are concerned. Mine is accurate with or with out handloads. I am very pleased with this calibre.

"borrow" some reloaded ammo for testing. I have seen big differences, and whether or not you decide to reload, you should know. make sure fore stock doesn't touch anything either. that has caused me stringing with factory ammo.

I LIKE A BROWNING, A-BOLT STAINLESS W/ BOSS
CHAMBERED FOR A .375 H & H

You ever tried the weatherby loads themselves. My brother and Cliff both have a 300 and I will tell you what I have seen my brother shoot and have seen cliff shoot tight groups out o 500 yds. Cliff used to reload his own but he tried the weatherby shells and has never got them grouped as well. For a box of shells is $40 something but well worth it.
They both are well pleased with them.

Why not take your old gun in and have it accurized, and maybe a new stock?

try this combo the sako modle 75 hunter in 300 whby. shooting whby factory ammo 180 grain barnes x bullets. I can shoot a group of three shots sand bagged on a bench with 10 min. between shots at 200 yards and cover all three holes with a quarter. that is good enough for me.with that result i know that combo works now i just have to make the shot when needed in the field.

I read this here already, but never the less it's worth restating. Have your barrel floated and your action bedded. Especially with the magnum cartridge, it will improve your accuracy with groupings many times over. Heat and friction along with vibration raise hell on good groupings. If you buy a new rifle, try to slide a $1 under the barrel between the stock, if it floats no problem, if it doesn't have the dealer get it floated. Don't believe them about laminate stocks not needing it, there full of it.