The honest answer is that it is going to be hard to add 10 oz. without using a barrel clamp or stick on barrel weight. If you add weight under the forearm you can usually add about 4... maybe 5 oz of weight. Let’s say you are able to add 5oz, that’s only going to have a center of mass around 8 -9 inches in front of the hinge pin. To add 5oz more to make it 10 oz you would have to add that 8-9” behind the hinge pin (assuming that’s the balance point right now, but it’s probably close.) That means that you are going to add that weight around the grip area, not all the way in the butt of the stock. If you add that weight to the butt of the stock it will move your balance rearward. So if 10 oz is what you want to add, you should probably add 5-6 oz in the butt and then add a 4-5oz weight onto the lower barrel at the location that keeps the CG the same. If they are both 5oz then just put them both the same distance from the current balance point.
Briley, Graco, and other companies make barrel clamp and stick on weights, but they may be for 12 gauge barrels only. If you want to avoid barrel weights and stick it in the forearm you may only ba able to add 6oz to keep the balance point the same.
Good luck! Let us know what you do and how it works. This will probably come up again and good or bad reports help the rest of us out!!
I have used the brass barrel weights under the forearm and loaded/sealed a fired 410 hull with lead to place in the buttstock to achieve the +/- desired weight while maintaining the CG. Depending on the amount of weight applied under the forearm will determine the amount and placement of the "lead weight" in the buttstock...trial and error. Good luck.
When I add weight I'll use tungsten crank shaft weights. Can get them in all sorts of sizes and they are about 60% denser than lead so they take up a lot less room than trying to fit lead weights in somewhere.
Once saw a Model 12 in Colorado Springs at a Pikes Peak GC Shoot that weighed 12 pounds according to the owner. He had the usual weights under the magazine tube, in the stock, in the mag tube with the feed disconnected. One thing he had I had not see before, was weights mounted on both sides of the rib, just ahead of the receiver. Not sure the M12 weighed 12 pounds, but it sure was heavy. May some CSP members can provide some info about it.