7 twist is real fast. It could be used for subsonic loads. You wouldn't need it to be 30 inches long though. That would get real long especially if you put a suppressor on the end of that!
You could likely shoot a 6 or 7 caliber length 338 lathe-turned banded solid bullet with very high BC. I know that twist is pretty good for the 295 grain GS Custom bullets.....