Having a hard time wrapping my head around this..5-6 rounds stuck in the barrel of what looks like is from a semi-auto of some type.  You'd think after the first one he would have been like .

Im pretty sure that one way or another I would have noticed something was amiss...no holes in the paper or other sign of bullet impact, rounds obviously would have sounded underpowered and how did they even cycle the action?

The barrel appears to be from a Thompson Sub Machinegun.  That could explain the 6+ bullets Sharps 1874 says are in the barrel.  

Speculation on my part- a squib load still had enough back pressure to cycle the action, reload fresh cartridges from the magazine and fire until the barrel burst.

He may have been shooting at an object that did not give clear indication of a bullet hit, thus did not know the bullet left the barrel.

Most of the ringed barrels I have seen over the years have come from squib/no powder loads in a rapid fire situation, the shooter couldn't stop their trigger finger in time when they heard the off report.

SWAG