Are you reloading for .223?
If so, then Sierra recommends the following for the heavier bullets:
69 grain : Not less than 1:10", which means 1:9" is OK as are 1:7", 1:8". These bullets work very well out to 300 yards. In this case, "less than" means a slower barrel twist, or higher ratio.
77 grain : 1:7" or 1:8"
80 grain : 1:8"
90 grain : 1:6.5"
Their lighter 50-65 grain bullets will work in any of these barrels. The bullets I've listed are HPBT MatchKing, and Sierra has put a great deal of research into what they publish. If you are using bullets made by other makers, please pay attention to their recommendations for barrel twist.
If you are not reloading, pay attention to the packaging for the ammo, it should (repeat *SHOULD*) advise you about the barrel twist. If it says nothing, I think you can assume that it will work in a 1:9" barrel. But as
EdF702 has indicated, avoid the really slow barrels.
I think that the most common combination seen is a 1:9" barrel shooting 55-grain commercial ammo, and that's a pretty good combination. Just pretty good, not really good.
If you really want accuracy, a 1:7" barrel shooting a 69-grain Sierra HPBT bullet is capable of amazing, one-ragged-hole at 100 yards accuracy, by comparison. You are not likely to see that with the 1:9" shooting 55-grain FMJ. The 77-grain and 80-grain bullets extend your reach, to the point where the 80-grain was the standard for 600-yard slow-fire when I was shooting NRA High Power Service Rifle.
Six Hundred Yards over open sights. Only those with good eyes need apply.
I got carried away again, sorry!
Chris