Home Depot carries a soldering paste called solder-it. It is low temp and very easy to use. However I would probably take a square graver or a punch and upset the steel in the dove tail to make tiny "teeth". That probably will solve the problem.
I had a troublesome range day earlier in the week when I found the front sight on a rifle, after having a wandering group, was loose enough that I could shift it by hand when it was in place. I still needed to drive it out with a punch, but there was maybe 1/16" of side-side play in the sight while it was sitting in the dovetail. Would the best fix for something like this be as simple as shimming with a piece of paper or flat silver wire to give it some extra squeeze in that dovetail? I have a second rifle with a sight that I need to adjust (a Parker-Enfield replica from the 1970's) that I've replaced the front sight on because the non-adjustable sights were far off for windage. Since this gun will have a dedicated bullet and powder load I was wondering about soldering the sight in place to prevent it from being able to become loose over time. Would the low (410 F) tin-copper-selenium solder used for copper pipe fitting work for this? I was thinking of using a soldering iron to heat the sight/barrel dovetail just enough to melt a bit of solder to hold the two in place. Is that low temp. and the use of a soldering iron rather than a torch going to be alright for the health of the barrel? Thanks for looking, good luck out there this this season!
What? No LocTite?*