the cap was blown off and basically looks how a cap looks after is been fired.
So possibly something caused the cap to detonate, right? Generally means contact with the recoil shield at some point on a chamber that is not in battery, right? On my one and only chain fire I do not remember finding the cap from the offending chamber. I wish I had paid more attention.
So a nipple not completely turned into the chamber, or a nipple that is slightly longer than is correct for the pistol, or a cap that was not pushed onto the nipple far enough. Or, more remotely, a fragment of cap got wedged between an unfired chamber's cap and the recoil shield.
Different nipple sizes in one revolver is not as uncommon as one might think. For those of us who remove the nipples every time the revolver is cleaned it is not hard to imaging a nipple that does not get completely turned in. Obviously it should not happen and is easy to avoid.
I use a little push stick to force my caps on but I can't say that I spin the cylinder through each position to check the relief between the cap and the shield at every location on every chamber.
On discharge, the cylinder comes back, the cap on a non-battery chamber bangs into the shield and Wham! In a perfect world this kind of thing should not happen but who says a chain fire occurs in a perfect world?