That was a funny story on a UT vid a couple of years ago.:
Federick Axelson explaining that they tried many materials and shapes for insterts to quiet the Wildcat. Once they tried everything they could possibly think of, they thought it would be a nice joke to leave the shroud empty...
Lo and behold it turned out to be the most quiet shot report they reached!
When I got my .177 wildcat mk 2, I did not know the gun was so loud. The stackable moderator is not included with the gun. Since I also own a bobcat mk1 unregulated, I borrowed the airstripper, and shroud from that gun, and is now quieter. I am planning to get a 1/2" adapter, so I can use whatever silencer I want on the wildcat shroud.