[krunck]

Please try this at home. You all have rusty iron ball laying around, right?

[fer]

It wouldn't be weird for someone in the south of France to have a rusty pétanque ball.

[Terr_]

Now you can be hoisted not just in your own petard, but also in your own pétanque.

[ece]

This clearly calls for thermite petanque.

[gorgoiler]

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?

[jcims]

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.

[xattt]

Joining the thread to see where iron balls can be sourced.

Some vendor on AliExpress? Those Lucky Iron Fish?

[dgacmu]

This might work with old iron dumbbells (or modern iron kettle balls if left unpainted). Your gym might hate you for it though.

[lstodd]

I'd guess exact form wouldn't matter. Wrap a hammer in foil and beat up something rusty.

[mhb]

Cast iron pan left outside for a day or two.