threes things pop out to me.....
fit a slip on rubber butplate...just cause it eliminates the recoil factor
get someone else to shoot a group for you...someone who normally shoots well with thier own rifle...eliminates the YOU factor
borrow another scope of known pedigree.....you DONT NEED TO ALTER ZERO..... so can take out of mates rings,screw into yours for test and then he can put it back,morethan likely wont shift his zero by more than fly pooh...good excuse for him to check zero......eliminates the "scope is faaarked" factor
there is little in this world more frustrating than a scope that wont hold zero,you can never trust it and will waste a lot of ammunition trying to sort rifle when its scope causeing issue.