Jim,
How well hide glue fills gaps depends on how thick you mix it. However, a thick joint will not hold. As someone else said, hide glue is brittle. On my last restoration, I had the situation you described where the forearm had shattered as it shrunk around a barrel that had been inlet too tight. I had a terrible time getting that thing back together. I could either fit the inside or the outside, but not both. I chose the inside because it fit almost perfectly there. You have seconds to get the piece in place before it sets and you have squeeze out that you can't get before it hardens. This filled the gaps on the outside. I used De-Glue Goo to clean off the excess. I also used a linen patch on the inside of the barrel channel to stabilize the whole thing a bit more.
Since restoration work is supposed to be reversible, you really don't have too many choices other than hide or fish glue. You use epoxy or cyanoacrylate on guns that are in service, but I don't believe it is appropriate on antique guns unless there is no other way to make the repair. Of course, many times, it is the lesser of two evils that you must choose, particularly since many owners want a gun that looks new or is functional.
The reversibility of hide glue is also useful in situations like I described above where it is difficult put everything back together correctly. I put that forearm together and took it back apart at least three times before I was satisfied with the job. Only the wax was removed in the process. You could tell it has been repaired but it was a good bit neater job than I had started with where white glue and superglue where used to excess in an attempt to hold the thing together in the past. It took a very long time to get all that old glue off without damaging the finish. I used a lot of tooth picks and q-tips.
Mark
