It's built on a M21 Beretta pocket .22, given a 1/2"longer than normal barrel, the gun is 5.5" long, so the silencer adds 3.5". It has to be sleeved, an underchamber added, and run "wet" to make it suitably quiet (about like a very mild handclap) The slide has to have a cap of steel welded to its top rear, so that you can add an extractor, a real rear sight and so that the slide has enough mass to not blow open too soon, letting out a lot of noisy powder gas with the ejected casing. Such a .22 makes a superb trail gun. I also carry a fine pocket 9mm, in case I need more power than the .22 offers. The pair weigh less than 2 lbs and the burden is split between left and right side carry, so it's quite unnoticable.