"rifle" reciever to pistol question

[squirrel_slayer]

did some more reading on the atf's web site this afternoon and I can't find anything saying you cant do what I'm wanting to do. I did find that a weapon over 26" oal and 16" barrel (rifle) or 18" (shotgun) yet no stock are classified as a "firearm" the pistol grip only shotguns (as sold from the manufactuer) are "firearms" I was however unable to find if making a "rifle" into a "firearm" was lawful/unlawful