I tested my factory 5r with several different factory loads, including premium rounds, and the ones that shot the best, consistantly, were the Winchester Super-X 150 and 180gr Power Points. Worked for me, as they were the cheapest things on the shelf and has a great reputation for a factory round.So I worked up a drop chart for the 180 grains and shot a few boxes, killed a couple deer too.
3-shot groups averaged 1", sometimes a little more, sometimes a little less. Out at 400 yards I shot a couple 5-shot groups and was getting right at 4" with a 1/2 value wind. Even with that, I had a bit more dispersion vertically than I did on the horizontal, so that points to velocity changes out the barrel, even though I didn't use a chrono. Hopefully I can tailor a load when I get back from overseas, and hope to hold 1/2 moa.
No mods except I took 1/2" off the stock and mounted a Limbsaver Recoil pad. With the weight of the gun, scope & mounts, I had no problem shooting a couple boxes through it from the bench. I really like that Limbsaver recoil pad. The factory stock (made by HS Precision) had several "imperfections" on the aluminum bedding blocks, so I'm pretty sure a skim bed would have helped it out a bit.
When I was doing research before I bought mine, It seemed the groups people were reporting for the 308 were around 1/2 moa, and for the 300wm between 1/2 and 1". Mine showed just that, right at 1" with no mods, no muzzle brake
and shooting the cheap rounds.
Hope this helps.