Yes it is DGH. It's like a dance hall in there. Not like any wood stock I've ever seen where there is a 1/2 to 1 inch hole bored down the middle for the receiver nut to go onto the receiver. It's just like a shell of a stock, very thin plastic and very open on the inside. I could probably stuff a thin raincoat and lots of other stuff in there. On other wood stocked guns I have I do as you are thinking with the shot, lead dowel or put in some kind of a deadmule or mercury type mule. Actually I find a proper length of lead dowel seems to work just as good as any type mule I've ever tried of the same weight. I may have to try the wheel weights. It won't cost to much to do that. I just need enough weight to take the edge off the recoil. I'm already using 7/8 oz shot loads but just need a tad more re-coil reduction so I can shoot more than 4 rounds a week.
Thank you from California.
Not sure how the Mossberg stock looks or the thread of the stock bolt but check out the Beretta 390 stock weights by Graco. I have one in a synthetic stock to balance the gun and it couldn't be any easier. Just screw it on. Hope this helps. Good shooting, Rey