Not crimping in a wheel gun can cause a round to go off in an adjacent cylinder, especially mixed up, high milage casings. I use 231 in my .44 and its easy to double load.(most likely the reason) A stuck bullet can do it as well. Did the fourth round recoil?...Did your buddy hit the paper with the fifth shot.
I think about 18 plus grains of 231 will come close to or exceed 36 000 psi. Cast bullets at 240 grs should be loaded with 6 grs of 231 not 9.1. I've had two guns explode in my hands in 25 years. Funny, thing is both were revolvers and i was at fault, due to improper loads...maybe get another powder that you can't double charge. Oh, i quess you need another gun first.

If your not sure about a load...get your wife to try it first.