one thing i tried years back on a sporter, it shot ok the first 2 or 3 shots, i tried the slight forend pressure with some shims, it helped, then i waxed or lightly oiled the bottom of the barrel and but a bead of silicone seal about 1" long in the stock let it set and it helped more than the shim, might be worth a try, if you don't care for the results just peel it off.

chet