Since you are making a connection, Are your checked bag(s) and firearms checked through to your destination?
My 2018 trip originated out of Dulles.
No real problems with firearms getting checked......However.....make sure at the check in counter that the person checking your firearms inserts the firearm check card into your firearm case.
Here's what happened:
I arrived about 2 1/2 hours early, when check in opened I was 2nd or 3rd in line, asked the clerk about my firearms, stepped to the side, a second clerk and I checked my firearm, and he asked me about ammo, I replied it was in my checked bag in lock box per TSA/ airline regulations, no problem. I then asked about the check card for my firearm that was to be enclosed in and with my firearm. Clerk said, it's okay we checked it, for me to go ahead lock it, pointed to the TSA counter for me to go, while he would escort my firearm over to TSA.
TSA: Okay, you're good.
Me: proceeded through passenger check in, rather quick, not much of a line, maybe 2 minutes. Didn't get 15 paces/steps other side, when my name was called to return to the TSA counter.
I identified myself, was asked/escorted into the TSA section, ask to open my gun case, and asked where is the firearms check card? I explained what had happened at check in, that the previous agent said, I was good to proceed to my gate?
This TSA agent handed me my firearms case and escorted me over to the check in counter, he then consulted with the fellow that was present and checked my firearm. The clerk's supervisor was called to the counter, and I received my firearms checked card that remains with and inside the firearms case. This took all of less than 10 minutes.
The TSA agent took the firearms case and we went back to the TSA area, the TSA agent along with a 2nd agent checked to insure no ammo in the gun case, chamber or mag, had me replace my firearm into the case, lock it, and told me, you are okay to proceed on.
Less than 20 minutes for this whole event.
Unfortunately by this time, going through the ticket passenger only mess, took about 25 minutes or so, as the lines were long and slow.
Everyone was friendly, helpful, and respectful, toward me.