Well it wasent on a double rifle, but a nitro muzzleloading pistol I built on a SAA frame with a novel cylinder built fron scratch. Like a fool kept stoking it up until I had 1200 fps from 158 grain head over 9.5 grains of unique. I flashed off 3 cylinders simultainously.

Dident hurt the gun much except for blowing the loading gate off.

Couldent wear that pair of trousers again though :-)