There is growing interest in silent, or at least very quiet, sniper rifles. Police and armed forces snipers now have a range of .22 caliber (5.56mm) rifles with built in silencers that fire sub-sonic (no “crack” sound supersonic ammo make) bullets. These are accurate up to 50 or 100 meters (depending on the target). Other weapons use larger and more powerful rounds that are modified so they are sub-sonic. Some of these sub-sonic rounds are for calibers up to 7.62mm. All these quiet bullets are mainly meant for special operations missions where stealth is essential for as long as you can maintain it.
Since the 1990s the lowly .22 caliber LR (long rifle) round has become a favorite among snipers. Professional assassins (usually thugs working for organized crime) have long favored using .22 caliber (5.56mm) pistols for their work. While not a powerful round, if you shoot someone up close with a .22 caliber pistol several important things happen. First, the victim is dead if you shoot him in the head, which is what pros usually aim for (as these guys like to say, "two in the head and you know he's dead.") Second there is hardly any sound if you use a silencer and not much even if you don't. Finally a 22 caliber pistol is small, even with a silencer. That makes it easier to conceal and easier to dispose of. For all these reasons special operations troops have apparently been using .22 caliber sub-sonic round weapons more frequently, as they have proved very useful for quietly killing numerous sentries, including watchdogs, at night as well as disabling lights and security cameras.
In the 1990s the Russians noted how Chechen snipers were effectively using .22 LR (long rifle, them little bullets kids use to hunt squirrels and rabbits with) weapons. Inside towns and cities, the .22 LR sniper was very effective, especially since the Chechens would improvise a very workable silencer by putting a plastic bottle on the end of the rifle's barrel, with a hole in the bottom of the barrel for the bullet to exit. Using a cheap scope, Chechen snipers were very deadly at ranges of less than a hundred meters. Such ranges were pretty common in built up areas. And since you usually did not hear the shot (to the head or face, of course), you had a hard time finding the shooter.
After suffering from these low tech .22 caliber Chechen snipers throughout the 1990s the Russians did something about it and in 2004 presented their own professional .22 LR sniper rifle, the SV-99. This is a little heavier (at 3.8 kg/8.3 pounds) than your usual .22 LR rifle but is built for professionals. It has a heavier barrel, a bipod, silencer, and scope. It's a meter (39 inches) long and can accept five, eight, or ten round magazines. There is a compartment in the butt stock for two five round magazines. With the SV-99, at a hundred meters, a skilled shooter can consistently put all rounds in a 12mm (half inch) circle. This is a specialist weapon, most likely used by commandos. But any trained sniper can quickly adapt to using it. And snipers like not being heard. Since 2004 a number of other small caliber (usually .22 inch/5.56mm) silenced sniper rifles have appeared and apparently sold quite well.