your buddy comparing the SKS isn't being fair. 7.62x39 doesn't hurt in any config i've encountered.
A metal buttplate mosin with surplus 7.62x54R can smart a bit!
I too put on a limbsaver brand pad on my mosin, and it made it a lot more fun to shoot off the bench. ONLY the limbsaver helped. others did not.
As for shooting standing up, I can negate a lot of recoil by shooting with a form that allows the barrel to rise significantly as I shift weight to my rear foot.
my sub2000 in .40S&W hurts my shoulder more than most rifle cartridges, and I cannot for the life of me understand why that is. it's tiny by comparison,. but it feels like someone using a small ball peen hammer and just smacking my shoulder bones. ouch