Gun shop owners sometimes say some stupid things from fear of liability. Many years ago I took a girlfriend to a big sporting goods store to get her some ammo for her new Ruger SP-101. I wanted to get her some .357 as well as some .38+P for it. Just wanted a box of each, thank you very much.
But Mr. Gun Counter Manager felt the need to lecture us (me) on the legal liabilities of using .357 due to the potential of it crashing through walls and killing neighbors all around. To the point of acting like he wasn't going to sell us a box of .357. Last time I spent any money at their gun counter.
I bought a rifle last year. It is in 5.56MM. The box says it is .223. The serial number on the box and the firearm are the same.