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The grey is carbon/lead residue. You can wipe that off with oil/CLP or use a pencil eraser. The plastic on the light more likely than not didn't melt but essentially got "sandblasted" by the particulates in burnt gunpowder.
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This. I doubt you could possibly load the gun fast enough to fire it enough to actually melt the plastic, let alone harm a metal tube.
Fouling blast it? Absolutely. Kill it with shock/recoil? Possibly. Get the lens dirty enough it's basically useless without cleaning? Absolutely.
Wipe it off with your choice of oil on a rag and don't worry about it.
If you do kill it, streamlight will swap out a new one. I know for fact their handgun lights are not built for 12ga