These shells were used for 'blowing' the plug on open hearth furnaces in steel mills. The plug is what held the molten metal back until tapping time. They are almost always a steel shot charge, akin to 0000 if memory serves. They are loaded to a very high pressure, and I'd venture to say would open up any sporting gun.
Whether the cases can be re-used, I couldn't say. They were also loaded in 4 bore cases.
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