Hi Joe,
There is certainly nothing wrong with soldering them, but it seems like overkill to me. Look at some barrels on muskets and see how thin the barrel walls actually are. Pretty thin. It would take a massive amount of corrosion to compromise the barrel if this is a standard octagonal one. But if you are really concerned, one option might be to get some really thick grease and pack it into the gap, much as you would do if properly packing a wheel bearing. Only force it in from one side until you see it coming out the other, and then stuff in some more. Most wheel bearing grease will stand up to the temp of boiling water, but I know this would not be a historic solution to the problem. Another solution which might be more historically correct would be to use sealing wax, like they used to use when sealing correspondence. Pitch could also be another historically correct solution. The problem with soldering is you have to get the barrel pretty hot to do a good job of it, but it can certainly be done.
As far as tapping in the dovetails, it makes a pretty tight connection, and if you have ever had to remove one, it takes quite a bit of force with hammer and punch. So unless your barrel is mighty thin, I don't think I would worry about it personally. If it were my gun, I would probably choose the sealing wax, since it would prevent the water issue. Of course, the barrel has to be hot for this too, but not nearly as hot as it does for soldering, unless you are using a very low-temp solder.
Matt
