Well, I've been developing loads for my custom .300 Win Mag-
Hand-lapped M700 action
26" McGowen #8 1-10" stainless barrel
XLR Evolution chassis
Vortex Viper PST
6-24X50 FFP EBR-1 MOA
PRW rings, Badger Ordnance 20 MOA rail, Caldwell bipod.
Now, I couldn't find any brass for .300 in my area AT ALL, so I've been using el super cheapo Federal 180 grain softpoint factory ammo- buy 'em, burn 'em, reload. It's actually cheaper than buying brass alone!
However, I've got good, consistent loads with both 208 Hornady HPBT's and 210 Berger VLD's, and neither will group at or under half MOA consistently.
It's not the shooter, bullet, or powder (Retumbo) that's responsible, and it certainly shouldn't be the gun! So that leaves my brass, and being the cheapest money can buy, I think it's a reasonable explanation.
TL;DR- Since I can't find any brass, can anyone tell me what factory ammo generally has the best, most consistent brass?