A couple of quick notes, from a topic I am somewhat knowledgeable in and have extensive personal experience. A suppressor with normal ammunition will reduce the sound by about 20db and might make it difficult for anyone in the area to figure out exactly where the shot came from. With subsonic ammo you can expect about another 20db reduction and this time, the actual action of the firing mechanism will actually be louder than the shot to anyone near the firing location. People away from the shooter usually react to the sound but are not immediately convinced it was a shot. It often sounds like a car backfiring, or a wood pallet being dropped on something hard, etc.
Benefit of a suppressor:
Mask shooter location
create doubt that a shot had or had not been fired
wonderful reduction in recoil, giving very nice, reliable accuracy
suppresses flash and environmental giveaways(shooting creates a flash at the barrel and/or a puff of dust from the ground and escaping expansion gasses from the barrel)
adds some reliability to automatic round feeding in many guns due to reliable back pressure.
In real life a cheap and mildly effective suppressor can be made with aluminum chips and two pipes of different diameters. A professional grade one can be made with 2 pipes, and a variety of washers and baffles arranged for proper series of expansion chambers which would delay and redirect the expansion of gasses away from the barrel. In cataclysm unless the character was skilled in engineering and machining he would only be able to create a somewhat mildly effective silencer. I LIKE THE VERSION WHALES IS USING A suppressed gunshot will still draw attention to the area, just not you directly necessarily.
Also, the best rounds for suppressed firing are the .22 as noted previously and the .40sw. These rounds are great due to lower velocity even before using subsonic rounds. A silenced .40sw carbine is an excellent urban weapon.
Again, I like the system Whales is using. He's simplifying it to keep it fun.