Well, doing my first build and ordering parts from several vendors. Barrel vendor won't be ready with stock for 2 months or so, so I don't have to pick the barrel yet. Going with the RRA upper/lower combo.. Anyway,... a.) I'd prefer the 18" or 20" barrels, vendor I like has 1:9 twist. What is the best weight 5.56 bullet to use for this configuration that's good for match, mil., and L.E. use? Am I looking to cover too many bases with 1 upper? b.) I see folks saying there is a difference between 5.56 NATO, .223, and .223 Wylde. What's the differences? (These barrels I'm looking at now are 5.56 NATO, according to the manufacturer.) Thanks for your educated insight.