In a revolver (yes, I know, not really a "pistol" but a revolver but many are sloppy with language), the short blows gas out the cylinder gap and makes a lot of noise.
In a fixed barrel gun, semi auto or single shot... depends. Longer barrel less noise.
I was shocked the first time I fired "standard velocity" long rifle from a rifle. LESS noise than my air gun. (Old Benjamin BB/pellet.) I didn't need hearing protection indoors. Finding standard velocity ammo is a problem except with those who supply target shooters. Much preferred for competition. You might want to hunt up some ammo of this sort and test it...
As said, the BB/CB ammo is much lower in noise. Colibri below that. Also less in power. No big deal for rats, but much drop at distance... Another challenge to use. The old Remington 550 semi auto (with the Carbine Wms. Floating chamber) will semi auto .22 CB shorts but not longs. Gap is covered. Find one of the latest models (from the 1950s) with grooved receiver and really terrorize the fur balls. Luck.