Modification of firearms

Question
I reside in New Mexico U.S.A.. I recently purchased a 12 gauge single shot at a flea market. It has been shortened to a total length of 27". The barrel is 18.5" from breech to muzzle. Is this legal, and if so, can I trade or sell it?

Answer
According to the National Firearms Act of 1934, your gun is legal without a tax stamp. To fall under Class I, the gun have have a barrel of not less than 18" and have an overall length of not less than 26."

It is not illegal for you to own a gun that is shorter than that, but it must be registered with the ATF as a Class III weapon, you must pay a $200 tax stamp for the privilege, and you must agree to impromptu ATF inspections at any hour of the day or night.

Unless there are local laws that forbid it, there is no federal bar from your trading or selling the gun.