Mike George
With a conventional over-under shotgun it is usual to put a lead weight inside the stock bolt hole, under the butt pad.

Make sure the weight is firmly attached to the butt pad, because if it slips down the hole you will have a hell of a job to get it out next time you wish to remove the stock bolt.

With a stock that doesn?t have a bolt hole, you can take off the butt pad or plate and drill a hole for the weight.