Recently I was on the phone with a helpful sales person at a good LGS to ask for details on a couple used Colt AR-15s they had for sale. The sales person mentioned that the receivers were stamped .223 and thus couldn't safely use 5.56 ammo. I'm not a AR guru, to say the least, but I said that I was surprised, wasn't aware that Colt made ARs for .223 only. He assured me they did (and maybe that's actually the case?). In any event, later I was in the store looking over one of the used Colts and a different--and very helpful--sales person pointed out the .223 stamped on the receiver and advised that the rifle should only be used with .223 ammo. I pointed out where 5.56 was stamped on the barrel, asked if that indicated the rifle could be used with both. Apparently he hadn't noticed the barrel stamping. He flagged down another sales person, asked him about the .223 on the receiver, and the second fellow, too, said it meant the rifle was limited to that caliber. At that point, I showed the second fellow the 5.56 on the barrel, whereupon he also was unsure of the significance, wondered if the original .223 barrel had been switched out. Finally, a third sales person chimed in and stated confidently that the rifle shoots .223 and 5.56. As I said, this shop is very good. This little incident doesn't fundamentally change my estimation of it. All three of the salesmen have no doubt forgotten more about ARs than I know. This incident, and others described in this thread, probably just go to show what a challenge it is to find and train staff to deal with the technical and legal complexities of firearms.