Well, this is a last ditch method if all else fails. I have a Braztec .410 I use for load development because of it's uniquely falty extractor. This extractor is not a perfect fit to the bore and leaves a bulged out rib like seam along the brass head on the bottom of any shell fired in it. This bulge is of very consistent size when measured with a micro-meter on a factory pressure load. Make up some reloads using reliable published data and you get an almost (within a couple microns) identical size bulge. Make the load a grain hot or a grain shy and you get a pronounced difference in the bulge size which is abvious by direct visual observation much less a micro meter corrosponding to the resulting pressure difference. Go a notch hotter and it splits the brass head right where that bulge usually forms. Thus I have a way to check my crazy out in left field homecooked loads for correct pressure with fresh hauls. Yes, I do use an armored safety enclosure and fire the gun via a string during such testing. Anyway I didn't want to drill and tap the gun and so weaken it for use in such testing, and yet wanted to mount a sight for using it in the feild.
Okay, enough gabby background story. Take some 400 grit wet/dry sand paper and carefully sand off the finish of the barrel by hand for about a 1/4 inch wide strip about 4 inches long on the top of the barrel. Next take a Weaver rail and place the sand paper over the barrel with the rough side up. Run the Weaver rail back and forth over the sand paper until the bottom of the rail now perfectly matches the curvature of the barrel. Finally use "JB-Weld" to attach the Weaver rail to the spot you sanded on the top of the barrel. The spot that was sanded should be slightly smaller then the Weaver rail so that you get full coverage with the "JB-Weld" and their isn't any bare metal left exposed so it's still rust proof. Mount your choice of scope on top of the Weaver rail. My prefrence is a $40 Walmart BSA red-dot. Oh, yah use the original full strength "JB-Weld" that takes like two days to fully cure and not that much weaker "JB-Quick" stuff. I've beat the h*ll out of my mount and it's stayed rock solid.