That’s at a store, but they do that for every gun purchase, even breech loading shotguns. What about private sales? It’s just a semi-auto rifle. “Assault rifle” doesn’t have a clear definition. You don’t need a background check to buy/sell a semi-auto 22 rifle privately, or a Ruger mini14, or a semi-auto shotgun. So why would this be different? I think you are mistaken. If however I’m wrong, and you have a valid source please share.
Handguns sold between private parties must use an FFL, this is not true with long guns.