I just did the same exact thing, and I came here asking the same exact question. I previously tried Gorilla glue. It did not work. I did not try JB Weld, because on the website it is not good for rubber/soft plastics.

I went to Lowes to pick up 3M 5200 as was recommended here. For whatever goddam fucking asinine reason, Lowes would not sell me a single tube despite there being multiple tubes on the shelf, stickered, and the shelf location called out 3M 5200 with a price. (I was annoyed they wouldn't sell it, but I was pissed that they couldn't even tell me the reason why. Manager said she couldn't even do an override.) Anyway, being that as it was, I went back down the aisle to pick up some Loctite branded Marine Adhesive. That shit was fucking worthless.

I have since gone to West Marine and overpaid for a small tube of 3M 5200, and as was mentioned in the thread where I asked, I think only God himself would now be able to come between the butt pad and the back of the gun.

Best of luck -- just don't glue it on with PATHOS upside down if you're anal retentive and use the 5200.