From the breech.
according to the batfe the barrel leghth is measured from the face of the (closed) bolt. if you stick a wooden dowl rod down the barrel and mark where the end of the barrel stops,pull it out and measure with a tape measure it will give you the proper barrel length.
If it is not marked on the barrel, you should have a gunsmith measure the chamber length.
The BATFE website will give you the federal answer for barrel length and overall length. Whether a state is more restrictive is something you will have to find out from each state.
There is no restriction on barrel length but the overall length of the firearm must be 26.5 in or greater
18"
That would depend on the make and model of the shotgun in question,and the length of the barrel/barrels that came with the shotgun.
A shotgun barrel length or rifle barrel length starts at the point where the rim of the case touches the shoulder of the chamber and extends to the tip of the muzzle. In most cases, if the barrel is threaded on, yes it would include the threads. lcj
Muzzle to bolt face
If you mean the legal length of the barrel? it is 18 inches and for rifles it is 16 inches.
A little over 36"
Close the shotgun action and insert a rod down the barrel to the standing breech (to the rear edge of the chamber or where the back of the shotgun shell would be). Mark the end of the rod protruding from the barrel. Remove the rod and measure its length. Per federal law (GCA 1934), this must be a minimum of 18"