Just flew w/ handgun for first time last week...
...went through all sorts of worry for no reason.
I used a pelican 1510 case, which is a hard-sided case designed for carry-on use (for carrying cameras and such)... I loaded a GPS, two boxes of ammo, a few other odds and ends (knives, flashlight, and so on) into the case, and slapped a standard (NOT TSA) padlock on the outside. I actually had the gun in a second small padlocked case just because that's how I usually transport them... so it was locked inside a locked case. As such, if the TSA had wanted they could've asked me to remove the outer padlock.
I declared the firearm at the check in counter (DFW) and the woman basically said "Just tell the TSA bag screening people..." and then went on to chat about how busy the airport had been earlier in the day and so on. At the bag screening checkpoint they asked "any firearms, chemicals, etc." -- I pointed to my 1510 and said "this contains a firearm and ammunition. I then pointed to my other bag and said "this is just clothes and so on."
The screener took bags and boarding pass, and pointed me to a row of chairs a few feet from the screening area, saying "Please wait there while we screen your bags." I asked them if they were going to want the key to the case, and the guy said no, not unless there was an issue. I didn't have to do this, but frankly I was just carrying a $140 revolver I'd purchased w/ my C&R while visiting Texas and I can't imagine anyone wanting to mess with it, nor steal it... and I wanted to see what their reaction was.
A few minutes later the screener came back, handed me my boarding pass, and said, "Thank you Mr Myname, everything looked fine -- have a good flight!"
In other words, it was a total anticlimax. Upon arrival in CA my bags popped out on the carousel along with everyone else's. BTW: I had one of the "indicating" TSA locks on my non-firearm bag and they didn't open that either. Someone (I think it was the TSA but I wasn't paying that much attention) marked the tag on the gun case with a red grease penned F, and put a matching mark on the claim tag, but that was the only difference in handling between the two bags.
It's a bigger deal to you than to them.