Images of Tyler Brown, the Cambridge, MA shooter this week, clearly show that he is using a short-barrled AR-15 variant with a "pistol brace".
I will reiterate my position that this is not a handgun, it is a short-barreled rifle, and such weapons ought to be banned for the purposes of ordinary self defense in populated areas. These devices exist solely to circumvent the public safety protections of the National Firearms Act of 1934.
Obviously, Brown does not care to follow the law, regardless.
