It wouldn't be weird for someone in the south of France to have a rusty pétanque ball.
This clearly calls for thermite petanque.
Now you can be hoisted not just in your own petard, but also in your own pétanque.
Certainly a ball peen hammer that could be encouraged to go rusty for scientific purposes. I wonder if it would work well enough striking a sheet of aluminum foil on a hard flat surface?
Hammer faces would work (def wear goggles!!!). Hit the face of the disposable one with a mixture of hydrogen peroxide, vinegar and salt and wait an hour.
Joining the thread to see where iron balls can be sourced.
Some vendor on AliExpress? Those Lucky Iron Fish?
I'd guess exact form wouldn't matter. Wrap a hammer in foil and beat up something rusty.
This might work with old iron dumbbells (or modern iron kettle balls if left unpainted). Your gym might hate you for it though.