

Everyone’s telling you why “It doesn’t happen”. They’re not objectively wrong in their answers of how resilient firearms can be, but they’re also not answering the question.
The ultimate answer for a lot is “broken down and recycled”. How do they get there, though?
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A lot come through “buyback” programs, where guns can be turned over to authorities for some nominal reward. These tend to harvest a lot of inoperable weapons, frequently from people who had one but didn’t know how to otherwise get rid of them.
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In states with more lax firearm laws, scrap dealers may accept repairable weapons as scrap metal. In more stringent states, they may only accept them if you’ve destroy the weapon as /u/SolOrion@sh.itjust.works outlined in the ATF poster.
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Even in states with strict firearm laws, guns can frequently be turned over to authorities without charges. (CAUTION: Read guides on how to do this, and consult your local laws and policies before treating this as truth. Better yet, consult a legal professional.)
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In some rare cases, a gun dealer may accept the gun, strip it of useful spare parts, and sell them independently.
At this point, the gun will be deliberately damaged to render it nonfunctional (if it isn’t already) and sent to a scrap metal handler. Metal components will be melted down and reused. Plastic or wood components may be recycled or thrown away.
Worked briefly in the waste management industry. Guns in the garbage were rare, but a problem. Policy was to call the local police to wherever they were found and turn them over. Police would take perfunctory statements from facility staff and review camera footage to verify someone hadn’t dumped it and claimed it “found”, then take the gun.
The real problem is we weren’t supposed to touch it until police showed up, so the garbage just had to kind of sit there waiting for them.