K-Romulus tipped me off that possession of a standard capacity magazine was not a crime in Maryland. I just couldn’t buy or sell one there. I looked it up:
After June 1, 1994, It is unlawful to sell, offer to sell, transfer or receive any firearm magazine (except a tube magazine for a .22) that will hold more than 20 rounds. Possession is not prohibited.
I’ll be damned. That’s good to know! Fortunately, even in Pennsylvania, I tend to shoot the scoped AR with the smaller magazine, just because it helps it clear the bench when shooting off my sandbag. That basically means nothing I’m transporting through MD is illegal. Now if only I didn’t have to stop at the MD/DE border to put the Glock away.