I have never checked a long gun, only handguns.
I have seen quite a few procedure changes over the years.
Initially, you signed the card and it went inside the hard case, then it was locked into your checked bag. Next you were escorted to the TSA person, who rifled through your bag (pun intended) and sent you on your way.
Had an agent once that insisted I needed two checked bags, one for firearm and one for the ammo. He "let me proceed" that time with one bag.
Latest procedure on United this week was at check in they looked at the case, card on the outside of the hard case. Case locked in checked bag and went to TSA where it was checked without me present. Presumably, somewhere in the bowels of the system.
Have had TSA test my firearm and it showed up positive for explosive residue. Second check showed explosive residue again. Had someone finally show up and said, "Yes, it might show up positive for residue, it's a firearm".
I made the flight, but they took so much time my bag didn't. In Lubbock, TX and they got the bag to me quickly without too much stress.