Hey, that revolver looks familiar!

Sounds like you diagnosed it correctly. A primer does have enough power to launch a bullet into the barrel and then stick it. The unburnt powder says you didn't fail to load it up, so dampness is about the only thing I can think of off the top of my head that would cause it. Water likes to stay in the primer cup and the flash hole. I usually don't use wet-cleaned brass for at least a week after tumbling it and then it stays spread out on a wire rack to dry. My preference is a two-week drying time in a good environment to be sure.