Using suppressors with bolt-action rifles
Firearm suppression is done best when used with bolt-action rifles. There are several reasons for this. First, longer barrels help reduce the amount of pressure as the bullet travels further before leaving the weapon. Second, the only egress point for gas is the muzzle. The slide or bolt does not automatically cycle thereby exposing the ejection port like other rifles or pistols.
When an AR-15 is fired suppressed, there are 3 sources for noise:
1. The muzzle
2. The ejection port
3. Sonic crack of the projectile
The muzzle blast is controlled by the suppressor and therefore cannot be improved much. The ejection port will most likely cycle as the weapon was designed. The subsonic crack will more than likely be present unless using subsonic ammunition. So you see there are many things needed to make an AR-15 quiet, but in the end it's expensive and still not very efficient.
A bolt action rifle such as a .22 caliber is a perfect host for a suppressor. There are several reasons that make it much better than an AR-15 or even most pistols:
1. A .22 contains very little gun powder.
2. Subsonic ammo is cheap
3. There is no ejection port
A bolt action .22 will be very quiet even with supersonic ammo like .22 LR. Using subsonic ammo only makes it better. You will not have the cycle issues with subsonic ammo in a bolt action because the weapon is not designed to cycle automatically. The shooter must manually work the bolt action to eject a spent cartridge and load a new one. Suppressors for .22 caliber is much cheaper than other calibers and some can even be used with variations of the .22 like the 5.7x28mm or 17 HMR.