Is it illegal to carry a 9 mm?

Answer 1

The question is so vague as to be meaningless. The answer depends very, very much on the details of where, how, and what exactly a "9mm" is (9mm handgun? 9mm rifle? 9mm assault rifle? 9mm machine pistol? something else?).

Answer 2

The legality of carrying a 9 mm firearm depends on the specific laws of the jurisdiction you are in. In some places, you may be required to have a permit or license to carry a concealed weapon, while in others it may be prohibited altogether. It is important to familiarize yourself with the laws of your particular area to ensure you are in compliance.