The other thing you could do is make the dovetail a bit wider by tinning the edges with silver solder Clean it off the top and bottom, and your sight will be a wee bit wider. But, like the other guys said, I'd try the peening route first. That seemed to work so the rest of this is for the next time, and there always seems to be.
On my current build, I had 1 barrel lug that was a wee bit loose, First I peened on the barrel, and then staked it in 4 places, just using a sharp chisel edge so I hit both the lug and the barrel with it. That did the trick. Since it's the bottom of the barrel it won't show anyway.