If anyone here is ever in Albany NY they have a great museum there. They have a whole room dedicated t0 9-11 and have quite a few artifacts, pretty sure it's the biggest collection of 9-11 artifacts. They have one of the fire engines that was partially destroyed, parts of the building and one thing i remember was a display of a bunch of guns recovered.
Just went searching for a pic and found this video and you can see some of them around the 2:20 mark. I seem to remember others as well, for some reason i'm thinking there was a pile that was melted together. You can see some that don't look like service guns so maybe there was some Govt. agency in the building that had them?