When I made my first rifle I drilled the hole for the tang screw in my drill press but countersunk the hole with a hand drill. When I tightened the tang screw my barrel moved up abut .010. I enlarged the counter sink, same problem, I tried different angles, same problem.
By now I had the hole in the tang so wallowed out I had to have it tig welded up so I could start over.
The next time I drilled the hole in the tang I replaced the drill bit with my counter sink and countersunk the hole at exactly the same angle as the hole I drilled, problem fixed.