Can such a device decrease the decibel level of muzzle report appreciably?
In short: No
In long: Nooooooooooooooooooooooooooooooooooooooooooooooooooooo
Such a setup would at best net you a few db, you would need sound equipment to notice the difference though. If you're curious and want to try out the theory, do it with a lawnmower or other small engine since there's no NFA tax for that. You will quickly find out why there is always some sort of packing or baffles in the can or muffler.