So I'm new to the AR world (some may recall my earlier first post about lower receivers), and I have a question/am in need of a memory refresh on barrel twists and bullet weights. As I recall from Basic Training, there were two barrel twists in these rifles: a 1:7 and a 1:9. (I forget which version had which twist rate. Also as I recall, one twist rate would fire the 55 grain bullet well, but not the heavier 62 grain-it would tumble pretty quickly after leaving the barrel, but the other would fire both pretty well. Does any of this sound familiar? So I have my lower, and I'm getting a A2 upper with a fixed handle, which 20" barrel (20" is the right length for a regulation USGI rifle, isn't it?) should I get to have the most versatility? (Plus I guess I should be wanting to shoot the cheaper 55 grin ammo.) Any reason to buy one over the other? Thanks in advance.