This is Why You Must Be Very, VERY Careful When You Reload

This used to be a Marlin Guide Gun in .45-70, before it KABOOOOOOM!ed

My guess is either the wrong powder, or a double-charge. I lean towards the double-charge. Smokeless powders don't take up much room in cases designed for blackpowder. There is no evidence of a plugged barrel.

The shooter suffered relatively minor injuries to his left (supporting) arm from metal fragments. He's OK.

And damned lucky.