I ran out of time on that last reply, just noticed it went through right as the computer cut off, (thought it hadn't).
Any way, another idea besides a silencer, is to combine the shot noise with another noise that sounds more industrial and common in populated areas.
Like Maybe a gas engine (chain-saw sounding toy model motor etc.) or small power tool, loud electric motor etc....Then BLAM following/during shots isn't as obvious. (backfire, tool noise, breakage???)
A rotary tool like a dremel at the 28-30000 RPM range setting with a metal blade about 5 inches long on a metal plate backing gets pretty loud GRRRR VrRRRRRR GRRRR... followed by a pop probably wouldn't cause much alarm as just the bang alone. (careful the metal isn't going to fly apart from centrifugal force, also poor balance can mess up the bearings)
Like when workers are jackhammering I am always thinking Dang, "The possibilities"
Really though, I was demonstrating to a friend that a .22 doesn't cause much alarm anyway even in a very populated area, by smacking .25 nail gun blanks with a hammer in the middle of a street. (Raised no attention at all)...interesting.
Also with the rubber baffles sandwiched between metal, there is a safety advantage that if the projectile strays a little (or is fired from a smoothbore), it probably won't stray enough to smack the metal washers with much larger hole , and still make it through the next rubber washer with smaller hole than projectile diameter.
BTW, There was a production chinese pistol very similar to diagram of the English one above only with rubber baffle design.
The baffles were replaceable too.
Americans (and probably others) should keep in mind that possesion of all specific parts to a silencer (or whatever), Is the same as possesion of the assembled part.
So, ...the noise generator idea might be better IMO. 8)