Purcussion revolvers and revolving rifles were prone to chain firing. This is more than one cylinder discharging at once. If you had your hand and arm out in front of the cylinder, supporting the barrel, you might lose more than a burning coat sleeve! Proper firing protocol involved supporting the rifle by putting your off side hand under the trigger guard, like a two hand hold on a pistol.
(Incidentally, I once owned a Singer .45 that had been accurized. It would hit a dinner plate all day at 50 yards, if I did my part. I wish I still had it. On the other hand, I once had a Colt sear break at the range, and suddenly had a machine pistol going, until it ran dry!)