Unfortunately, the .308's only come in 24, 18, and 16.5 in barrels and they are not fluted. You may be able to get them as an option. I have all 3 versions and they shoot really well! 308 should do all you want to do anyway.
I feel the extra barrel length is needed for the big mags. 4 inches of barrel should yeild another 200 fps with the 190's. We shall see.