I've read that several times and still don't get it
My brother-in-law had a sawed off shot gun. They measured 20 inches from the firing pin (is that the *face of the breech*?) and cut the barrel. He got stopped for DUI and the shot gun confiscated. The FEDs measure from the end of the longest shell that the gun will take - In other words the length of *the barrel* DOESN'T include the receiver.
He spent 5 years in a minimum security federal penitentiary in Millington, TN for having that gun.
The good news was that while he was in the pen the time ran out on his driver's license suspension for the DUI....
*They* seem to take that fairly seriously, btw.