I know this will vary gun by gun.
Then what answer can be given other than to restate your question as a statment?
It will vary by gun.So, what's the most accurate factory ammo you've found for your 300 Win Mag?
Why does that concern you or in anyway help you in choosing the ammunition that will shoot the most accurate for you?
Tri-age your issue.
Start with the least invasive (ie expensive model avail.)
What factory 300WM ammunition is the least expensive, most avail and more importantly appropriate for the game you intend to use it on?
BUY SOME and shoot it.
then if you are unhappy with the accuracy of that load increase the cost factor until you find a load that you feel allows you to hit your target in the vitals at the distance you intend to use it.
Anything else is superfluous.
You have posted the question in the hunting forum, so i am assuming that you are going to use the rifle for big game hunting. So the ability to hit an 8" circle from field position at 100 yards would be sufficiant, right?
Most ammunition, most rifles are more than capable of that.
Most shooters are not.