Suppressed weapons are generally still semi-loud, but they also bring advantages such as greatly reduced muzzle flash. Suppressors reduce the sound of gunfire by allowing the rapidly expanding gases from the firing of the bullet to be temporarily diverted or trapped in a series of hollow chambers, known as expansion chambers. The trapped gas expands and cools, and its pressure and velocity decrease as it exits the chambers. Suppressors tend be very effective when used with subsonic rounds like the .45 ACP because such rounds do not produce a supersonic crack when fired. But a suppressor used with 5.56x45mm NATO or any other supersonic velocity ammunition will only reduce the sound of the gunshot by about 50% (the gun will still be as loud as a jackhammer). The biggest advantage of using suppressors is hearing safety. When using suppressors, earplugs are not necessary in outdoor shooting, allowing one to hear zombies more easily. However, hearing protection is still recommended for indoor shooting -- especially if using supersonic ammunition -- as prolonged exposure to noise at the decibel of a jackhammer in confined spaces where the noise have less places to travel other than to one's ears, can still cause hearing damage. Some direct impingement gas-operated weapons will run dirtier with a suppressor, leading to heavier carbon deposition cleaning the gun after a firefight is strongly recommended. Also, some weapons may have issues when firing suppressed -- they may not cycle correctly. However, such weapons may also have a "Silencer" mode (usually a simple screw that can be turned by hand), allowing them to cycle correctly.
Chances of one finding a suppressor in a zombie outbreak are minimal. Suppressors are typically used only by military and law enforcement special operations units, and civilian shooters with extensive collections.
Legal regulation of suppressors varies widely around the world. In some nations, some or all types of suppressor are essentially unregulated and may be bought "over the counter" in retail stores or by mail-order as they are considered a great help, along with hearing protection, to preserve the hearing of the user and any onlookers. Typically, in countries where suppressors are unregulated, firearms themselves are strictly controlled (by U.S. standards).
The United States taxes and strictly regulates the manufacture and sale of suppressors under the National Firearms Act. They are legal for individuals to possess and use for lawful purposes in thirty-eight of the fifty states.
Suppressors are frequently improvised to varying degrees of success, but even the best improvised suppressors wear down quickly -- most can only handle one or two rounds. This makes them impractical for most usages against zombies, which are rarely settled after only a few rounds of fire. One famous example is Vito Corleone's towel suppressor in The Godfather II.
Suppressors can fire supersonic ammunition which produces much better range and power than using subsonic ammunition. Older suppressors may wear down quickly when using supersonic ammunition, but most modern suppressors, such as those made by Quicksilver Manufacturing LLC or Thunder Beast Arms Corporation, are designed specifically to be used with supersonic ammunition; these suppressors are typically solid tubes without any replaceable parts, and tend to last very long -- it is not uncommon for them to outlast the firearms they are used with. Usage of a suppressor requires ensuring one's ammunition type does not conflict with the suppressor type. If one's ammunition velocity exceeds what the suppressor is designed to work with, the suppressor will fail to suppress the flash and sound, and may even damage the suppressor and the gun. Some suppressors require subsonic variants of common ammunition types, which are not commonly available for market reasons previously discussed.