A suppressor, also commonly known as a silencer, is a device attached to a firearm to reduce the amount of noise and flash generated by firing the weapon. It is a cylindrically-shaped metallic tube that is fitted into the barrel. Most suppressors work by allowing the pressurized gases in the barrel to slowly expand before they exit the muzzle of the firearm, resulting in a "phum" sound rather than a bang. The process is comparable to slowly opening a soda can and hearing a "ffft" sound rather than a "pop". Some suppressors are also structured like motorcycle mufflers in order for the sound waves to cancel themselves out as they reflect off of the sides of the casing. Such suppressors are inherently more difficult to produce since they require more precise cutting and assembly. For this reason, they are often bigger and are mounted on large-caliber rifles where the added sound suppression is needed most.

When mounted on pistols and submachine guns, the effective noise reduction of a military grade suppressor is astounding. On the Heckler & Koch MP5 and Mark 23, the weapon's noise is reduced to the point that the click of the action is louder than the muzzle burst. On sniper rifles, and large caliber assault rifles, the noise reduction is still immense, but a distinctive whistle is still present. Another important factor in sound signature suppression is the muzzle velocity of the ammunition. The MP5 and Mark 23 chamber 9 mm Luger and .45 ACP rounds, respectively, both of which are available with subsonic muzzle velocities. In large caliber assault rifles and precision rifles, the bullet itself produces a noticeable crackling sound as it travels through the surrounding air. For this reason, it is more difficult to effectively lower the sound signature of these firearms.

One solution is to lower the muzzle velocity of the cartridge. Some suppressor designs do this by allowing gas to bleed out of the barrel before the bullet exits, others do it with rubber "wipes" that use friction to slow the bullet. While this is effective, it dramatically reduces the range and stopping power of the cartridge. In addition, the wipes in velocity reducing suppressors that use them tend to adversely affect the accuracy, since they come in direct contact with the bullet after it leaves the barrel, and the wipes generally wear out and lose effectiveness after only a few shots.

Oftentimes, a subsonic cartridge may even be more powerful than a suppressed high-power weapon because the bullet mass may be increased to make up for the energy lost by lowering the velocity. A good example of this is the .300 Whisper cartridge, which is formed from a shortened, necked up 5.56 x 45mm cartridge. The subsonic .300 Whisper fires up to a 240 grain (16 g), .30 caliber bullet at about 950 feet per second (290 m/s), generating about 480 ft·lbf (650 J) of energy. By comparison, the standard SS109 62 grain (4 g) bullet at 950 feet per second (290 m/s) would generate only 125 ft·lbf (170 J) of energy, only as much as a .22 Long Rifle cartridge. Since the .300 Whisper is based on the 5.56 x 45 mm cartridge, firearms such as the M-16 rifle can be changed to .300 Whisper by simply changing the barrel.

Most suppressors can be removed by unscrewing them off the barrel, but others, such as suppressors that bleed off powder gasses to reduce the ammunition velocity, are built into the barrel and can only be removed by removing the barrel. Integral suppressors are also more robust than detatchable suppressors, because they attach to the barrel over much of the barrel's length, making them less susceptable to bending if subjected to torque, such as when the firearm is dropped. A detachable suppressor that is even slightly misaligned can come into contact with the bullet, which will significantly degrade the accuracy at best, and at worst can result in the suppressor being ripped off the firearm by the impact.