From
http://www.triplebreakproducts.com/FAQ/twist_rate_for_barrel.htm
Twist Rate for Barrel?
What is the right twist rate for an AR15/M16 barrel? The below is from Corey Sattler of Olympic Arms. It will give you a good idea of what you should look at.
* 1x7
Technically too tight of a twist for any .224 bullet widely available, including the 80 grain. Manufactured originally to stabilize SS109 and/or tracer ammo, but further military testing has shown other twists to be superior. Some of the very heavy custom stuff (87 grain?) may work well, but why?
* 1x8
Great twist for 69-80 grain bullets. Sierra once informed me that the perfect twist for their 80gr was 1x8.2xxxxx (blah blah blah big long engineers explanation followed).
* 1x9
Good all around twist ratio. Best suited for 52-69 grain, but either end of the envelope will be questionable.
* 1x10
Practically the same as 1x9, but favors the lighter side a little more.
* 1x12
Great for the 40-52 grain bullets. Most often found on bolt action rifles as their primary use is varminting. All right for the 55 grain, but not the best.
* 1x14
If you want to under stabilize the .224 bullet, use this twist. Useless except for tumbling effect it causes upon bullet impact. IMHO, better to shoot straight with a good HP bullet as your chances to hit are better and damage will most likely be greater also.
The bullet type would be more dependent on what you're doing, ie. target shooting (FMJ) or varmint shooting (HP) and what feeds best in your rifle.