Register? Unless you live in one of the few progressive states that are extremely anti-2A, there is no federal requirement to register non-NFA firearms, which probably accounts for more than 98% of the firearms in private hands, including semi-auto AR-15s, AKs, etc.
When you purchase a firearm from a licensed dealer, you fill out a BATFE Form 4473 in order for an instant background check to be completed. The licensed dealer is then required to keep the form for 20 years and allow the ATF to view the form at anytime on demand. Also, if you then decide to sell the firearm as a non-licensed dealer, there is no federal requirement to perform a background check, keep a record of the transaction, etc...and that's where the paper-trail stops.