I think I have done the same thing with a stapler

It's a revolver, it would just vent out through the cylinder gap.

But what about the air trapped between the stuck bullet and the incoming bullet? I have trouble believing that there's enough blow by in the rifling to let the bullets do that.

Good question. I think that the pressure could escape through the rifling grooves. It also looks like cracks were starting to form on the upper half of the barrel.