I've seen several references to ruptured barrels, especially with trade muskets. Samuel Hearne mentioned it, and attributed the problem to the natives' use of odd projectiles when they could not get proper balls. However, left hand injuries related to firearms accidents were not unusual on the frontier. Didn't Thomas "Broken Hand" Fitzpatrick get his nickname that way?
There are some old illustrations showing trappers with a spare ramrod carried in the rifle bore. This was not simply a convenient place to carry the spare rod. I can't speak from personal experience, but I understand that a patched round ball, in a rifle carried on horseback all day, can "drift" up the bore, creating a dead space between the ball and the powder charge, resulting in the same effect as short-starting the ball. A spare ramrod carried in the bore might help keep the ball in place, and would also allow for a quick visual check. If the ball had drifted, a smart push on the rod before withdrawing it would re-seat the ball and prevent possible barrel rupture.
Notchy Bob