I can name a couple if you are willing to convert models, and do a bit of work on the textures/animations.
For starters, Google Sketchup Warehouse has a HUGE collection of models, and I am sure I have seen a fair few guns on there. Some of them are pretty badly textured, but all of the actual models are brilliant. Its a great resource if you have a little cash to spend on the pro version and have conversion software.
http://artist-3d.com/free_3d_models/08military_weapons.php?s=0&srt=2
This website has a collection of military models, although they are all in 3ds Max. They all look pretty good.
[href]www.turbosquid.com[/href]
Just search 'Guns' and setting it to lowest price first, there are a fair few free guns there too. Once again, most need to be converted. I counted 734 free models, though I did it just by page numbers... So I'm assuming a fair few of them won't actually be guns.
I use 3d Object Converter 4.80. Its relitively cheap and the free version seemed usable. However, I use Unity, and I'm not entirely sure how it fares converting TO .x. However, it seems to be pretty good at converting from.
