buckslayer .
If you tell me. the distance from center to center of your scope rings and the exact distance to the target, you say 50 yards. Did you measure it?
I will tel you how much shim you neeed to elevate the front ring.
But you should first fire the gun again with the elevation adjustment in the center of its range. Then when you shim tyhe base you have adjustments both up and down and at the same time your x-hair will be in the optical center of th scope. That is the best place to have the x-hair with the least optical distorsion.
A rough estimate with the scope rings ctr to ctr of 4" you would need 0.100" under the front ring to raise the impact br 15" at 50 yards. That is a lot of shim. If the rings are 4.5" ctr/ctr you would need 0.112".
Something is way out of whack in either the scope or the rings or the barrel contour, and with that amount error the Burris Posiline rings will not have enough excentricity either, the largest ones the make are .020"
that would not even be halve enough.
Fred M.