Well, there's state registration and there's federal registration. Indiana has voluntary state registration. No one volunteers. It requires an affirmative act on the part of the buyer to initiate a government registration of gun ownership of gun G by owner O. No one ever bothers.
Otherwise, there's ATF Form 4473 - Wikipedia, the free encyclopedia . They have these for every sale that happened through a federally licensed gun dealer. When they start walking through their file cabinets full of 4473s and they come to yours, if you can't produce the associated firearm, you'd better be able to give them name, phone, address, and SSN of the person you sold it to, or else they'll just spitefully TRIPLE your federal healthcare bill.