Not from artillery, I can tell you that much.
I can think of ways of making ‘cast iron’ appear, but none of them are plausible in this scenario.
(Thermite reaction being one, but that’s highly implausible.)
It’s not even from Gerald Bull’s HARP project.
http://www.astronautix.com/articles/abroject.htm
http://www.astronautix.com/lvs/martlet.htm
http://www-istp.gsfc.nasa.gov/stargaze/Smartlet.htm