I've seen the crystal, but just assumed it was crystalized reaction product from eating the copper. Of course, such a crystal could cause corrosion if it is hygroscopic and the corrosion inhibitors are used up or gone.
Is the MC-7 ammoniated? I've never tried that particular one.
What sort of condition is the bore in overall? I'm wondering if some light firelapping would help. Or Tubb's Final Finish process. It should take the sharp edges off the pits. You can, of course, treat the bare bore with something like Sprinco Plate+ Silver before he shoots it again. That should also help keep stuff from accumulating so easily in the pitting. Also, if you are loading the bullets, you can treat them with hBN or add Tubb Dust to the powder.