Quote Originally Posted by Not_A_Llama View Post
Drop a dowel down the bore with the bolt forward. Make a mark on the dowel at the muzzle.

IIRC, that's the official way.

Shotgun also has to be 26" overall.

This post not to be construed as legal advice.
What about if you have a rotating bolt, like a Benelli Nova? That skews the measurement.