You're not saying you're attempting to take standing 300yd shots at elk are you?
At 300-500yds I'd just find a rest, and make it happen.
I'd recommend doing some push ups and just go about it that way, I really don't think a 6lb rifle vs. a 8lb rifle will be any easier to shoot offhand...
Weatherby is now loading some $39/box ammo in .257wby and .300wby I believe. So if you're dying for another weatherby with cheaper ammo, look at their site and these two calibers.
Get yourself a semi-lightweight setup in a .300WSM and call it a day. Or a .270WSM
Checkout a hill country rifle if you're looking to spend wby accumark money...or for another grand I'd look at HS precision. I think the hill country will shoot sub MOA they say, and the HS says 1/2"