I should probably clarify the question of barrel length. I'm after an 18" barrel or more, not an 18" specifically. 24" might be a bit much but 18, 20, 22, and 24 inch barrels are all in the ring.
ETA: I'd have to look and see what specs I've used, but I don't think any of my loads are in the 5.56 range. With that in mind, I think the Ruger with the 22" barrel is about perfect for the price and availability. A Savage would also be fine, but I'd really like the magazine compatibility of GI magazines. I have to decide if a left-handed rifle is really that necessary, as I don't see the Ruger in LH versions.