Thread: what glue to use...

Same as Marty said. What you're doing is similar to the bedding process. Rough it up and put some holes into the stock inner (not all the way through) so the resin has something to lock into. You could use JB weld, but since you have epoxy and balloons, that will work just fine. Make sure you leave barrel clearance, and dam the area with plasticine or clay so it doesn't flow where you don't want it to. Your balloon mix should be relatively thin to maintain strength and flow so it will fill properly.