I THINK where the original ATF agent got it wrong is this (please bear with me)

You know there is a tang sticking out from the breech end of the barrel, sort of like the tang that sticks out from the top of a 45acp barrel. This tang has an elongated slot in it for the breech face to lock up into. THAT (the rear portion of it) is where the agent measured from.

If you’re not Paranoid, You’re
not paying attention
Be a Sheepdog
JYD#105