I think that the organizers say that they can refuse to accept guns from some individuals, so if you bring in 20, they might not take them. But two or five....
From the first link below the picture:
“No firearm dealers or pawn shops are allowed to participate, and officers can limit the number of gift cards to individuals.”
Also, there is the possibility that they will run out of funds.
I doubt there would be any problem with turning in five...
Heck the magazines are likely worth $10 each. I would keep the magazines and sell them separately.