I just filled the buttstock of my Choate Tactical with sand last night, adding 1 lb, 5.6 oz for a total weight of 14 lb, 7.2 oz. I'd like it to weigh another couple of pounds, but I don't have any lead shot handy. You could do a sand/lead shot mix until you get the weight right. It would be much easier to remove than the expanding foam.
-Dan