Carrying a 10 round magazine plus one in the chamber is completely legal. Note, however, that possessing a post-ban large capacity magazine is illegal, no matter how many rounds you have in it.
That said, the late attorney Darius Arbabi told me that one of his clients was charged with carrying a large capacity weapon because he was carrying a ten round magazine plus one in the chamber. He got that charge thrown out (there were other charges as well). This was relatively soon after the MA assault weapon ban was passed, IIRC. Arbabi told me that the police and DA were angry at the defendant and were throwing charges at him and hoping something would stick.
I think the chance of getting arrested and charged for this today is very, very small and the charge would not hold up in court.
That said, the late attorney Darius Arbabi told me that one of his clients was charged with carrying a large capacity weapon because he was carrying a ten round magazine plus one in the chamber. He got that charge thrown out (there were other charges as well). This was relatively soon after the MA assault weapon ban was passed, IIRC. Arbabi told me that the police and DA were angry at the defendant and were throwing charges at him and hoping something would stick.
I think the chance of getting arrested and charged for this today is very, very small and the charge would not hold up in court.