You don't need to register it if it's in rifle form. You would if it ends up being a handgun.
Also, it's illegal to use a receiver that was sold as a rifle to make into a handgun per federal law, if I remember correctly. In other words, you can't buy (for example) an AR-15 rifle, and then convert it into an AR pistol. It all is determined what was used on the 4473 when it was purchased (rifle or handgun)