mmholum;
First of all, I am glad you made it through this without injury. It is not pleasant to lose a gun but you can make that back (losing a hand or an eye would have been MUCH, MUCH worse!)

I can only speculate. It appears to me that all three cartridges went off (common with a catastrophic accident). It has every appearance of a double charge of fast burning powder.

I have two 550B's and it is possible to have a double charge even with a progressive press with auto advance. I have seen it.

Send those picutures to Smith and they may want to take a look at your ruined revolver. If so, they will pay to have it sent in. Any thing that they offer to do for you will be out of the goodness of their hearts. They are not apt to offer to warrantee this loss (unfortunately).

It should be a reminder to us all, that we need to maintain "eternal vigilance" to avoid a similar mishap. It could happen to any one of us.

Dale53