Quote Originally Posted by Cycler
One simple way to check barrel length is to make up a dowel rod marked at the exact minimum length (16" for a rifle, 18' for a shotgun) and drop it down the barrel until it hits the closed bolt. If the mark is inside the muzzle, you are legal.
This is the post I was referring to in which the poster states to use this method on an installed barrel with the bolt closed.

Kevin