longer = faster but more nose heavy. A 14 to 16 inch will do well; as we have 16's in the ar's and they really aren't even 100 fps under my 20 inch. My 2-250 Stryker is a 14 inch, and it looses 300 fps or more, but it has more case capacity too. For what a 223 can do, I'd say 14 inch.