It is really hard to blow up a black powder barrel. I was involved with trying to blow up a barrel just to see how much it would actually take to blow it up. We double charged, doubled loaded, used way to much powder, short started the ball, and finally fired the barrel with a full load and a short started bullet. Using black powder, under controlled conditions, fired with a fuse from a safe distance, and we could not get the barrel to burst. Smokeless powder is another story. The barrel will blow up, no ifs or ans about it. This appears to me to be a burst barrel. In the old days, barrels were folded over and forge welded by hand, and this one could have been a poor welding job. The barrel gave way at the breech, the threads are for the breech plug to screw in. Charlie P figures the hole was for a flint lock vent, I think it could also be a where the drum was screwed in which would have made it a drum and nipple cap lock pistol. Imagine if the guy was aiming that pistol, and the breach plug gave way, it had to head straight for the guys eye. Also, the notch for the under lug pretty much settles it as a gun barrel. If it was a tie rod, threads would be on both ends.
View attachment 1097326This is a drum and nipple that would screw into the barrel. The cap goes onto the black nipple.