I am shooting a 300 win mag and my question is what ammo would provide the best accuracy at ranges over 200yrds(factory load). thanks in advance.

There's really no good answer to your question. First, depends in part on what big game you will be hunting to determine the best ammo/bullet weight to use. Following that, and assuming there will be several factory loads available for a given weight, the only way to determine what's best for your particular rifle is to buy a box of each brand,
and try them to see which one performs best in your rifle. Each rifle is an individual as to its' likes/dislikes when it comes to ammo.

Every rifle is different. What shoots good in one rifle might not shoot good in yours. You will need to try different brands and see which one shoots best in your rifle.

i agree with jeffs & Virginia7. every rifle is differant. and likes differant ammo. just go to the range and find out.

When I first started buying guns and got into hunting some of the gun shops in the area would sell shells by the round, might be a thing of the past but with the price of your ammo worth a try. If your local dealer was willing to do this you could walk out with 4 different brands 5 shots each for about the price of a box. Like you've been told only way to know is shoot them.

Thats a heck of an idea but I've never seen it done around here.

try them all