I've shot several hundred rounds through my old GPR with the same results. Then an old timer told me I should be shooting with the butt plate on the bicep, not the shoulder. All I can say is it worked for me. The hardest part was training myself to place it in the right spot every time.
Looking at your photo I was placing my butt plate 3 to 4 inches to the right of your location right on the bicep. Another thing for off the bench, or X sticks is I always place at least a finger, or a 2 finger hold right at the forward support position to keep barrel rise to the minimum. And help insure the recoil is straight back.
I was a doubter but tried it anyway. Was glad I did. Try it yourself and see. It can't hurt anything.
I'd also drop the powder charge until the problem is worked out. 85 grains of 3f is a snappy load and 105 grains of 2f can hurt, I know from experience.
That'll take some getting used to but I'll give it a try. Rather reluctant to drop charge, these are both very accurate, they shoot better than anything else I've worked up in this rifle and they are intended for elk which is my primary use for this gun. Guess I'll have to work up a lighter plinking load for the time being. I've shot these loads all day long offhand and on sticks etc and didn't have this problem. Heck, I have an 8 1/2 lb. 338. Win mag that doesn't smack me around like this did the other day
Thanks for the tips!
Are you shooting round balls, or conicals?
During the 70's and 80's, I shot literally thousands of roundball loads (enough to go through 20-25 lbs. of powder per year), I shot that rifle at least several times a week, and used it for competition, and hunting throughout the year.
My .54 GPR loved 90 grs. of Goex 2F behind a patched .530 round ball, and was my "go to" gun for EVERYTHING (elk, caribou, deer, coyotes, bear, and antelope).
20+ years of shooting a Lyman GPR, and I never once noticed any recoil.
When I lived in Alaska, I owned, both, a .338 WM and a .375 HH, now those things had some recoil but we don't talk about those types of guns here.