Bigwheels,
UNSC's greatest distance is only 600 yards. It is hardly worth using anything more than a 308 for those distances. I think that using your 300 Win mag for those distances is a waste of the barrel's throat, and alot of powder. To be sure, your load will have a tremendous amount more wind bucking ability than any 308 shooting anything from 175 on up, but I still think if it were me, I would wait until more distance is available.
However, if you want to bring it down, I could also bring out some rifles like that, and we could shoot the 600 yard matches where they have catagories for bench rest, hunter, tactical types of rifles. They shoot at 600 yards for smallest group, closest to center of target and so on. If you are interested, let me know and we can make a day of shooting for groups at 600 yards.
They have decent target pits and we would just take our turn pulling targets in the pits like everyone else. It might be fun just for something different. I could always put my ammo in a cooler to keep pressures down. My load is a 190 Sierra that at aboutr 45-50 F. travels 2960 fps, and is warm. I simply won't shoot it in hotter weather after I got some sticky bolt lift, and found that my velocity went 3000 fps. So, I figured rather than pull the bullets, I will use it as a cold weather load.
Gale McMillan built the rifle, and his recommended load for that bullet was 70 grains of IMR4350. My first 2 barrels went so darned fast, I reduced it to 68.5, and still get plenty of velocity. That 70 grain load was before I had a chrono...I shudder to think what the velocity was... Barrel #1 lasted 850 rounds, but I shot the rifle hot in lots of fast paced precision matches back in the early 1980's, barrel #2 lasted 975 rounds. I sold it to a friend, who had it bore scoped. The smith took a half inch off the throat and said it was beautiful inside. It was a Schneider 1-10 twist polygon bore...It shot fabulously. The guy I sold it to has been using it for a couple of years and says it is easy to get half minute groups with it. Barrel #3 went 1200. Barrel #4 is the one I'm on now. At 700 rounds I had it set back, and it has another 400 rounds through it since then, and it shoots great. #4 is an Obermeyer 1-11 twist 5R barrel with a MTU profile.
If you want to shoot the 600 yard groups match, check out Upper Nisqually's web site, and it will tell us when the matches are scheduled. It would be fun to try. They will even let you shoot a number of different rifles if you want.