Base screws that are tight don't mean the bases are tight, Bases should be installed 1 screw at a time to make sure the bases are being held tight by both screws!
I'm trying to visualize what you mean and it just isn't happening.
P
On more than one occasion I have had Talley base screws be too long. It will play hell with your groups if the very front one is touching the threads on the barrel. To check, remove the front screw and peer down the hole with a good light. If it's too long, you will see the shiny spots on the top of the barrel threads.
That will prevent the base from seating even if you tighten it gorilla tight. Trust me on this one............
