The Canuck gun has a hollow butt. Unscrew the swivel stud. It passes through the stock shell into the bottom of the recoil pad. With the screw removed, the pad swings out from the bottom. You could almost put a bag of #9 shot in there.
With a wood stock, I have filled aluminum cigar tubes with molten lead to make a weight to fit into the long hole bored in the butt.
If you want to get fancy, put a piece of spring between the butt pad and the end of the lead tube, to keep in pressed against the muzzle end of the cavity. On recoil, the buttpad hits you and then you get hit by the sliding lead. Much like the same way a semi softens the blow.
I did this in a sporter 300WinMag and it worked very well. I took it out when hunting.