Any secrets about what to put in a plastic gun butt stock to deaden the hollow sound beside spray foam.

Spray foam is the best bet. You can put a garbage bag or something similar in the void before you spray the foam in it. The foam will adhere to the bag and not the stock in case you want to remove it. It's not a perfect solution but it helps.

Pack it tight with styrofoam packing peanuts. They will do the job and they are almost weightless. Worked perfectly on my Kimber Hunter.

Yep that’s what we are hearing what to do. My buddy did spray foam & it didn’t get hard in the middle so the peanuts are better & easier to remove too thanks guys

Crammed full of Wally World bags.....

Sorry to hear about your buddy. 😢

Next time, tell him to go with the blue foam.

What's the option if the recoil pad is glued on?

You can do epoxy and microbaloons.....works for McMillan.

If you do foam it needs to be done in layers to be able to gas off.

plastic bags work good