Are handgun silencers effective

Answer 1

As effective as what they're attached to. To be completely suppressed, the projectile needs to be traveling at a subsonic velocity. The .45 ACP is a naturally subsonic round, as is the 9x17/.380 ACP. The .22 Long Rifle is borderline. Most cartridges are not, and require special loading to remain at subsonic velocities.

With a supersonic projectile, the sound will still be suppressed to some degree, but the crack of the projectile as it goes supersonic will be unavoidable.

Revolvers, contrary to what's shown in several Hollywood films (Desperado is one which comes to mind), cannot be effectively silenced. The gap between the cylinder and the frame allows for the escape of gases (and, consequentially, sound).