I'm having a 300RUM built also and it will also have a 1/9 barrel on it. I choose it because I plan on shooting the 200-220gr bullets and after talking to my smith we decided that would be a good twist. Later I was concerned that if I wanted to shoot some lighter bullets (who knows why) that it would be to fast for them. My smith said the 1/9 was a good compramize; it would stabalize? the heavier bullets but not over-spin the lighter pills.
I haven't got/shot it yet, but "that's what I heard."
Genises 27-3: Now therefore take, I pray thee, thy weapons, thy quiver and thy bow, and go out to the field, and take me some venison...