I travel at least 2x / month outer island (Hawaiian Air) and 4 - 6 times per year outside of Hawaii (mostly Hawaiian or United as primary flight with transfers at times). Hawaiian Air, I do the drive through check-in. Small locked case for pistol that goes in a soft sided suitcase / rolling duffel. I keep ammo separate from the locked pistol case (easier). I tell them the weapon is unloaded in its locked case and ammo is separate from the weapon. I will also tell them that I have under 11 pounds of ammo. Never had an issue at the drive through. Never had TSA ask to open the locked pistol case inside the suitcase but have had the TSA inspection flyer inside the suitcase. Now if I have a locked Pelican, my own locks, after check-in I go right up the escalator to the TSA area on that same end, for inspection.
On one occasion I did my check-in inside upstairs, drive through closed. I went to the customer service / check-in for Gold / Platinum / First Class. Had a new agent working there. She was confused about weapons in cases and declaration. She checked with TSA and she got the scoop, which matched what I told her. I also told her I had under 11lbs of ammo. She looked up the ammo restrictions on the computer, verified and actually wanted to weigh the ammo separate on the scale. Not a big deal, she was learning and trying to follow the rules.
Now for dedicated gun cases like Pelican cases, I often travel with Pelican cases. 2 are square style cases for gear, the others are standard rifle types. Now the Pelican cases draw different attention. I use my own locks (NON-TSA) locks which are OK. However after check-in, you must take those cases to the TSA area for inspection. They will swab the case and have you lock them in front of them. I have also had them inspect gear cases without firearms so I could put on my own locks, so they won't cut them open if they get curious later. This is not necessary, but if they want to inspect the case, they will cut the non-TSA locks. Porter will then take the case(s) into the back for baggage. I also note that it seems that if I am traveling with Pelican cases, my soft sided suitcases will guaranteed have a TSA inspection slip inside. If no pelican case, usually no inspection on the standard suitcases, even if I declared ammo only. I will also guess that if you have "military" or "tactical" looking suitcases, it seems that they get profiled and will often get inspection by TSA. My OD green rolling 511 suitcase / duffel gets hit every time, while other standard style suitcases get ignored. Pelican cases with no weapons / ammo, will get inspected.
I will note that when I go through the drive through, if I am not taking a weapon but if I am taking ammo only, I will declare the ammo, but the agents there never raise an eye at it and don't even ask any further questions. Maybe they know me well enough already?