A silencer that consists of a tube with two endcaps is a silencer in name only. It will reduce noise just enough to require a tax stamp by an unlicensed owner. In other words it would suck. I tried my 22lr can on my pistol with the baffles removed, it hardly made a difference. With just one bafle installed it made a big difference.
Given the same cartridge, a pistol can be much louder than a rifle, with or without a silencer attached. The longer barrel sucks out lots of heat and noise as the bullet moves down it. In other words, pistols are always louder than rifles.
Baffles are the key here. Mesh sucks too.
I am not impressed with the Metrogun extension on paper. http://www.metrogun.com/
How loud is the Metro Gun system? About the same as the sound of a car door closing, 72 Db at 100 feet, when shooting sub sonic ammunition.
The standard way to measure gun noise is to position the microphone one meter to the side of the muzzle (and in some cases the shooter's ear). Metrogun does it 100 feet away. 100 feet can change the noise intensity by a factor of 1000 or 30 decibels. When I wrote to the maker, he did not have any details on why this method was choosen and when I pressed him for details he told me I did not have to buy it.
I have no reason to believe that use of the Metrogun extension is going to eliminate the need for ear plugs, but that is what they imply when they say noise is reduced to 70 decibels. I would like to hear one though.