Does anyone know how to properly dispose of a propane tank like one off a gas grill?

I thought about lighting a match and watching it go, but something tells me thats probably not a smart idea

a gun out at the shooting range with a video camera, safest way i can think of

i think you should be able to take it to most like grocery stores or gas stations or whatever where you can exchange them. alot of the time theyll take and refurbish them

toss it in a river?

Take it to a welding supply store and, assuming it's in decent condition, they'll probably buy it off you.