I'm not to swift on diagnosing AR problems so I appreciate any help. I have two rifle length AR's (one is 6mm the other 556)...I use the same fixed lower for both. The 6mm runs great but my new 556 appears to be under gassed because it will not keep the bolt back on a empty mag so it also will not advance rnds from the mag. I have tried different mags...no help, tried different ammo...no help. Also made sure the 556 is lubed although it has a Nickle Boron M16 BCG. I do not want to change any buffer springs because the 6mm runs perfect. Would an adjustable gas block in the 556 provide more gas? Ideas? Help!