As explained- you have what is legally called a Short Barreled Rifle. A firearm made from a rifle. Possession of one (barrel less than 16 inches, or overall legnth less than 26 inches) is a serious Federal crime unless the gun has been registered with the BATFE and the $200 tax paid on it- BEFORE possessing it.
If you are in the US, it would be prudent to contact the ATF (they are on the first page of the phone book) let them know you found it, and want to surrender it.
You do NOT want to get caught with it. Trust me on that.
What we have here is... failure- to communicate.