I would like to know what is the optimum twist rate and barrel length to shoot Berger's heaviest 30 cal bullet in a 300 win mag. I believe their bullet is a 225 grain and is pretty long. I was thinking 27" barrel with maybe a 1:10 twist but I don't really know. Input please.

There is nothing wrong with your plan of 1:10" and 27".

Berger's website lists the necessary twist for each bullet. See if you can find some H-1000.

if that's your barrel don't mess with the 225 go strait to the 230 hybrids.....

Berger's calculator says a 1:10 twist isn't enough. Is 1:9 or 1:8 too fast?

No it's not too fast.

FYI

Of course you are looking at BC in hopes of making a hard hitting 300 Win Mag at considerable distance. I advise to run some numbers on the 210 gr @ 2900 fps versus the 230 hybrids at what velocity? Find some data on how fast guys are making it leave the barrel. There comes a point of diminishing returns when stepping up bullet BC and therefore length and weight. The velocity can be so low that you're making performance suffer. I'm not saying that is going to happen in this combination, but I would run some numbers in jbmballistics.com and see what it tells you. Not even concerned with vertical trajectory (drop) just concerned with retained velocity and retained energy down range as well as wind drift.

The "Reloading Depot" in Sniper's Hide probably has posts from guys that have tried various bullets and the end results.

Berger's heaviest bullet in 30 cal is the 230 grain Hybrid and OTM. The 230 Hybrid has the higher BC of the 2. It is a BADDDDDD bullet. It hits harder than any other 30 cal I know. I have run the 230 Hybrids and the Hornady 225 BTHP. The 225 BTHP requires a 1:11" twist. The 230 Hybrid is a longer bullet, and needs the 1:10" twist. I have run both bullets out past 1 mile in the 300 WM and 300 RUM. For ultra long range engagement, the 225 and 230 work very well. For your medium engagements (600-1000 yards), the 208/210's work well also shooting a little faster and flatter. But the 225 and 230 really shine out past that with the higher BC's. If you build a 300WM with a 1:10" twist, the 230 would be a good choice. The Berger twist rate calculator shows a 1.37 stability factor. The old chart used to say 1.4 was minimum, now it says 1.5 is minimum. But the 1:10" will work just fine for that bullet. It won't hurt to go with a 1:9 or 1:8, if you wanted.