I know a couple of folks who have gone to these events and watched what came in. From what I am told, lots of bb guns, pellet guns, broken guns, etc., are brought in. Occasionaly, old heirloom guns that bolonged to someone's deceased family member come in.
If we had them in my city, I would probably go by and see what I could pick up. Hell, I have a bunch of gift cards from Christmas. I'll gladly trade a McDonalds gift card for your deceased father/husband/crazy uncle's gun that you inherited and don't want anymore. Hell, bring me an M1 Garand and I'll throw in a trip to the nearest McDonald's and buy you a McRib. Bring me a nice old pistol like a 1911, Luger, etc., and I'll upsize it to a value meal.
Instead of building newer and larger weapons of mass destruction, I think mankind should try to get more use out of the ones we have.