From the muzzle to the end of the buttstock.
From the breech.
From the breech
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.
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.
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.
If it is not marked on the barrel, you should have a gunsmith measure the chamber length.
A little over 36"
Any shotgun with a barrel length of less than 18 inches and/or an overall length of less than 26 inches.
A shotgun is a shotgun, regardless of what you do to it (except should its length go below certain limits, it will be reclassified as a pistol or handgun in some jurisdictions). If you saw some of it off, then it becomes a "sawn-off shotgun", pending the above restriction on length.
18.5 inches
18"
minimal length of a shotgun is set by the Federal Firearms Act....The barrell can be no shorter than 18 inches(measured with the breech or bolt closed, breech or bolt fase to muzzle) with a minimum overall length of 27 inches.