And you can make your own 'dead mule'. Just get appropriate size tube insert a lead weight with a 'weak' spring behind it. Fill the rest with a viscous liquid. I think even differential lube would work (90wt). Could also be some sore of silicone, etc. Seal the tube and insert in the butt stock.

We used to fill motorcycle handle bars with lead shot and oil to dampen the vibrations in the bars.