101stCurrahee,
You could buy a new rifle in one of a couple of brands of CHEAP but, accurate rifles in 300 Win Mag, that are guaranteed MOA or better, that actually shoot sub-MOA, for under $550. I have several that shoot very nice groups with both factory 180gr Barnes TTSX loads and my Berger 215gr Hybrid loads. The BNIB T/C Compass 300 Win Mag rifles my buddy and I got a steal on (under $230 each, including shipping & FFL transfers) both shoot sub-MOA even in their flimsy tupperware stocks. I'm going to put mine in a Boyds Pro Varmint Laminate sometime this year. (I'd be glad to loan you this one. Its wearing a relatively inexpensive Leupold Freedom 4-12x40mm capped turrets scope, not suitable for your 500 meter range requirement but, has an EGW 20 MOA Picatinny rail stress free epoxyed and then torqued to spec, mount installed, that would work nicely for your Nightforce NX8 scope.)
The other inexpensive sub-MOA rifle I would recommend is the Ruger American Go Wild in 300 Win Mag. I now have 11 of the Ruger American Centerfire rifles, in various chamberings from 223 Rem up to 300 Win Mag. (Yes, I have a LOT of rifles!). Most of them are the Predator variant. ALL of my Ruger American Centerfire rifles shoot extremely well. The 300 Win Mag Go Wild version has a SLIGHTLY heavier barrel, camo tupperware stock, radial muzzle brake and burnt bronze cerakote finish. It runs BNIB for under $550 including S&H, if you shop around for it. It is also available for under $600 for a stainless version with the black tupperware stock and plain SS finish. I haven't fired one of these stainless versions, or even seen one at the range, so no idea on how well they really perform. Still, they have Ruger's MOA or better warranty.
Another route you could go is a nicer rifle, like a Sako, but that means some considerable cash, even for a decent used one. I've also had great luck (and some not so great) with older used Remington 700s, but would HIGHLY recommend a decent aftermarket trigger for it! Or buy a new or used Winchester M70 with the CRF (pre'64 style) action. I've had excellent accuracy from many of these, except for two 375 H&H Mags.
Before you guys go nuts with the 375s lack of accuracy, know that I have a Winchester M70 Safari in 416 Rem Mag that cloverleafs 3 round groups with Barnes 400gr TSX at 2,300 fps at 100 yards! I have shot as many as 50 rounds at a single sitting at the bench with that rifle/load combo. I also have a Remington 798 Laminate, in 375 H&H that shoots 1-1/2 MOA with my Barnes 300gr TSX load. So yes, I can shoot the heavy mags quite nicely, thank you. Those two M70 375s just haven't performed well yet, and never. I may have to rebarrel them.
101stCurrahee,
PM me if you're interested in borrowing the T/C.
Kevin