The issue was early revolvers occasionally had a problem with flash-over, which is to say when one firing chamber ignited the fire sometimes spread to adjacent chambers in the revolver's cylinder. With a pistol style revolver, held only by the grip behind the cylinder, the result was the gun might be damaged but the shooter was generally not seriously injured. With a long arm, one hand would normally be supporting the barrel in front of the cylinder. This had very different results in the case of a flash-over -- the flashed over chambers would typically discharge directly into the shooter's forward hand and forearm, causing devastating injury.

Accidents occurred, word got around, and after some early experimentation few people were willing to risk using a revolver style rifle. Eventually revolver designs improved to the point that flash-overs almost never occurred, but by that time alternative styles of rifles such as lever and bolt actions had taken over the market.