I have heard of powder drums blowing out before. Obviously, this can be
very dangerous for anyone standing to the shooter's right. Percussion guns with drums screwed in the side of the barrel work very well, but they need to be checked periodically for stability.
Just one guy's opinion here, but there is no way I would try to refit a drum in that hole. It just looks too big.
You have several options. One is to retire the rifle and save it as a wallhanger, and this might be something to consider if it is truly a valuable collector piece. I would be reluctant to glue the drum back in for appearance sake... Somebody may try to shoot it.
Another option is to cut the barrel off at the breech, refit the plug and tang if it's any good, or replace it if not, relocate the tenons, shorten the stock, and move the barrel back. This has been discussed.
Another possibility which I have not seen discussed in this thread yet is to cut off the barrel at the breech, and fit something like this:
View attachment 57928
I could not find a good photo of exactly what I was looking for, but I hope this one will suffice. The one shown above is a hooked breech, but it doesn't have to be hooked... it can be in one piece. It serves as a barrel extension as well as a breechplug.
The image below may illustrate it better, although the "PhotoBucket" banner obscures things somewhat:
View attachment 57929
The plug has a small-diameter "powder well" or chamber, and the drum screws into the side. You can see from the illustration that CVA came up with their own twist on the idea, but either way, this makes a strong breeching system, particularly for a relatively small-diameter barrel. This is actually much more secure than screwing the drum directly into the side of the barrel itself, even for a new rifle. You get the drum (or touch-hole liner for a flintlock) much more solidly anchored. This technique has been written up in the past, and has been used to salvage a few old guns and make them shooters again. You don't hear of it much now, and I expect it would require a very skillful gunsmith/machinist to fit it all together without having to modify the lock or stock, and I have no idea who might take on a job like this now. However, it has been done.
Respectfully suggested,
Notchy Bob