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I'm running a 16" .223 match chambered Walther barrel, so gives me good speed, ie 2900 fps with 65gn GKs.
Also running a 11.5" 5.56 chambered Colt barrel. Speed is down, only 2500fps with the 65s,...

One of my ARs likes the 75gn HPBT steel cased ammo.
You can get the projectiles and handload them too, otherwise the 65gn Gamekings are good, the Hornady 68gn HPBT and the new Tipped 69gn and 77gn...