Are there any brass mfg to avoid? Which bullet weight is the best compromise of BC/velocity? 26" 1:9 barrel. TIA.

My favorite brass is Norma and ADG. IMO, the best all around bullet weight for a "do all" in the 300 WM is the 208 to 215 weight range in the 2900 to 2950 fps range. Hornady has the 208 A-max/ELDM and the 212 ELD-X. Berger has the 210 VLD and 215 Hybrid. Those are hard to beat.

If you gear it for longer ranges, the 225 and 230 weights are good. I've run the 225 BTHP out to a mile in my 1:11" twist 300 WM. And it kills very well, also. The 1:9" would be great for the 225 ELDM, 230 Berger or 230 A-Tip.

I was in Africa recently doing a management hunt for a wildlife conservancy. There was a hunter from Denver there with a new Christensen .300 WM. He brought 185 grain Berger Classic Hunter loads. The first day he shot a mature waterbuck. The bullet did not penetrate the shoulder socket and had to be shot 2 more times to bring it to the end of its suffering. The next day they shot a gemsbok and it ran off with a crater in its shoulder. Never recovered. I guess they work well on whitetail, mule deer and antelope, but they did not work on 500 pound animals in Africa.

215 gr Berger hybrid — one shot kills on everything from WT to elk to big African plains game

this is what im running in mine. never had anything run more than a step and super accurate to distance.
for brass Im using mainly Federal Premium. Have some Hornady as well.

225 eldm, nosler brass, 75.5 h1000. 2880 fps
Some pressure in warmer months.
1:9 twist

Thank you all for the feedback. Rifle came with 50 rounds of FGMM and 10 pieces of brass but I was underwhelmed at the initial ~1MOA groups. I wasn't sure if Fed brass was worth saving. I picked up a box of 200gr Hornady to try this weekend but will shortly move on to a custom load for it. Since this is going to be primarily a long range rifle I figured something in the 210-215 range would likely be the best compromise, although if 220-230 can fly, let's do it. While I would like a bullet that could be used for hunting, if I give up a little on that side to get better LR performance that would be fine.

if you don't want that federal brass I'll buy it, pm me

The FC brass is soft and will wear out the primer pockets quickly. But a reload or two is fine.

Run the ballistic charts comparing the variety of bullets you want to play with. There will be a crossing point where the heavier bullets out perform the lighter one's. That mark is generally in the 600 to 800 yard range. So, if you are gearing your rifle up for beyond 800 yards, the heavier and higher BC bullets will have less wind drift and shoot flatter further out. The wind drift is the big factor I look at.