Imagine, if you will, James Bond sneaking silently down a hallway in an enemy base, hungover and sore from the previous night's drinking and unprotected sex. In his hand is his trusty Walther PPK, with a long black silencer protruding from the front. He sneaks up on a helpless guard, takes aim without any regard for those pesky sights, and pulls the trigger. The weapon emits a comically quiet PEW! and the guard goes down.
Everyone knows what a Hollywood silencer sounds like. The sound has become so well known that it's hard to convince keyboard warriors that it's not what a suppressor (the correct terminology) actually sounds like. After all, the effect has been used in everything from James Bond to Boondock Saints, and if it's so widespread, it must be right, right?
Um, no.
Surprise! Hollywood lied.
First things first. Yes, they're called suppressors. I've seen people get really mad because someone will correct them when they call the things silencers. They insist that there's no difference and that everyone knows what they mean when they silencer. Well, they're right, everyone does know what silencer means. It means that no one is going to take them seriously for the duration of the conversation. Gun nuts can be a prickly lot, especially when dealing with millennials whose only firearm experience comes from Call of Duty and Halo. Oh, and while we're on the topic, say it with me: magazine. Mag-uh-zeen. Not clip. A clip and a magazine are two entirely different things. The terms are not interchangeable, and I will skull drag the next person I hear mixing it up. [NOTE: The difference will be explained at a later date.] If the word suppressor is too unwieldy for you, it can also be called a can.
Moving on.
A suppressor is essentially a muffler for your firearm. If anyone who knows about both firearms and cars is cringing right now, just work with me. I realize that it's not a perfect analogy, but it's the best way to explain it in layman's terms. Anyway, it cuts down on the noise that results from the rapidly expanding gas leaving the barrel by allowing it to expand more slowly. Though they look like solid blocks of metal, the insides are mostly empty space, with a series of baffles placed at strategic intervals. Baffle designs vary wildly from manufacturer to manufacturer, but they all do more or less the same thing.
Now, a suppressor does not make a firearm silent. In fact, most don't even make them all that quiet. A gunshot is literally a controlled explosion, after all. They do, however, greatly reduce the noise, and they alter the sound so that, to the untrained ear, it might not even sound like a gunshot. They also almost completely eliminate the muzzle flash, which is one of the biggest advantages to using one in a combat scenario. Also, unlike what Call of Duty and Battlefield tell you, they can have a positive impact on accuracy and range. The extra weight on the end of the barrel can reduce muzzle climb, especially on pistols, and prolonging the contact with the propellant explosion can actually increase muzzle velocity, depending on the design of the can.
Suppressors come in a wide variety of designs, and can be found for nearly any weapon system you care to name. Yes, that includes shotguns. I've also seen them for .50 BMG rifles, though I've never personally witnessed them being fired, so I can't speak for their efficacy.
Exactly how much noise reduction one can expect varies wildly. The size and design of the suppressor plays a role, as does the caliber and type of weapon being used. I've seen some for .22 LR pistols that render the weapon no louder than a powerful airsoft gun. As the video I'm going to link below shows you, a .45 ACP pistol with the right can is about as loud as a paintball gun. Larger rifles and shotguns might be quiet enough to be fired comfortably without hearing protection, though it should be noted that there's a difference between what one person might consider comfortable and what OSHA would consider safe. It's also worth noting that I'm quite deaf, so my idea of comfortable and your idea of comfortable might be drastically different. I can fire an unsuppressed 12 gauge shotgun without hearing protection and not even have my ears ring any more than they already do.
Further noise reduction can be achieved by, as the video above calls it, "shooting it wet." In the video, the shooter adds a shot of silicon grease to the suppressor's baffles. I've personally only seen it done by dunking the can in water, but both seem to work fairly well. How it works, I haven't a clue, but it does. This video also effectively demonstrates "first round pop." The first shot fired from a suppressed weapon will typically be louder than subsequent shots. This is due to the oxygen in the suppressor igniting from the heat of the explosion. As the oxygen is reduced, the pop vanishes, and after a shot or two, the suppressor will function as advertised. Shooting it wet can greatly reduce first round pop.
For maximum results, use subsonic ammunition. A supersonic bullet creates its own miniature sonic boom as it cuts through the air. The result is a distinctive crack that anyone who has ever been shot at can recognize instantly. Subsonic ammunition does not have that crack, and as a result can be much more effectively suppressed. Though most factory pistol and rifle rounds are supersonic, subsonic rounds are available in many calibers. Certain rounds, such as most of the .22 family, .300 Whisper, and .45 ACP, are almost always subsonic. .45 ACP is one of the most popular rounds to used in suppressed pistols, because they have excellent stopping power and you don't need to special order subsonic variants.
It should be noted that any worthwhile suppressor design will allow the operator to take it apart. This is important, because the baffles tend to collect the carbon residue that results from firing. The more caked up the baffles get, the less effective the suppressor, and in extreme circumstances, the buildup can actually impede the bullet's flight path and cause it to tumble.
So there you have it: suppressors in a nutshell. As always, the comments and my inbox are open if anyone has any questions.