I did mine at MHT in NH and it was very easy. I called on New Years Eve to make and appointment and the office stated "Well we are open tomorrow". So Jan 1 I loaded my rifles into the case as I intend to travel with them and drove to the building USCBP and the FBO were located in. Left them in my truck and went to find the office. Not great signage but I called the number and the gentleman wandered out.
I had blank pages of the 4457 and my completed ones. He checked them over, we wandered out to my truck, he did say make sure to open the bolts before I grab the gun to check serial and seemed pleasantly surprised that I had pulled them already. He verified the numbers on the guns and scopes and we wandered back into the lobby to finish paperwork.
All told he seemed a bit bored and was happy to have someone to talk to on a slow day. We spent more time talking about my trip then the paperwork. The longest part of the official stuff was him having to check his app as he said he does not do a lot of the forms.
10/10 .gov employee interaction