I'm not sure what the motivation is to destroy functional weapons, but there is a legal definition of weapon destruction and your intended method doesn't achieve or approach it. In order for it to be legally destroyed in the U.S., the part containing the serial number, ie. the receiver*, must be rendered non-functional in such a way that it permanently cannot be returned to service. Preferred methods are smelting, melting, crushing, and cutting through with a torch. More
HERE on the ATF site.
"
An unserviceable firearm is not destroyed and is still regulated as a “firearm” under Federal law. Any method of destruction must render the firearm so that it is not restorable to firing condition and is otherwise reduced to scrap"
The ATF is quite picky. Why not give them away to a respective collector or firearms lover?
Bob
* Lower receiver on an AR-15.