I've seen / read references to this theory (?) before. I can't say it isn't true, but it seems quite a coincidence to me that I've NEVER had a multiple discharge with the chambers greased and the only multiple discharge I ever experienced was on the third (and last) cylinder full that I fired without grease or lube. This out of hundreds and hundreds of loadings in that same gun - 99.9% of them greased or lubed with beeswax.
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That goes beyond coincidence, and I remain convinced that it was because of those bare balls. (They were also none too tight of a fit in the chambers I might add - although they did manage to stay in place under recoil.)
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Of course that doesn't mean that loose caps can't cause chain fires too - just that it's never happened to me.
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Uncle R.