Everyone who has listened to gunshots in close proximity knows some cartridges are much louder than others. In the other direction, some cartridges are much quieter than others.
Everyone understands a .22 LR out of a rifle is much quieter than a .338 magnum out of a rifle.
Not as widely understood, loads for the same cartridge vary widely in how loud gunshots are perceived. The .22 Long Rifle cartridge is a good example. The highest energy .22 LR cartridges, such as the Aguila Interceptor and the CCI Velocitor, are much louder than CCI Standard Velocity, even though all are firing 40-grain bullets. Further down the scale, the CCI Quiet .22 load is much quieter with a 40 grain bullet than the Standard velocity. The 29-grain CB load is even quieter. The Aguila 20 grain primer-powered Super Colibri load is still quieter. It makes less noise than many pellet guns.
A key difference in how loud a load sounds is whether the bullet goes supersonic or stays subsonic. To be very quiet, a load must operate in the subsonic area. Projectiles should have an average muzzle velocity of 1070 fps or less to stay sub-sonic reliably. The speed of sound depends on the temperature. At 70 degrees Fahrenheit, the speed of sound is 1128 feet per second. At freezing (32 F), the speed of sound is 1087 fps. At 100 degrees F, the speed of sound is 1158 fps.
For a particular load in a particular cartridge, the length of the barrel makes a considerable difference in how loud the discharge of the cartridge sounds.
An Aguila 20-grain Super Colibri can sound loud out of a short-barreled pistol and quiet out of a rifle.
To have quiet loads in any caliber without using a silencer/suppressor, the key is to have a subsonic load fired in a long barrel. To reduce the noise of the gunshot, a fast powder works better, quickly building pressure to accelerate the projectile, then dropping pressure as the projectile moves to the muzzle. To reduce the noise of the shot, it is best if the projectile loses a few feet per second in the last inches of the barrel, with the internal pressures dropping and the gases inside the barrel losing temperature. The lower the pressure of the gases behind the projectile, when the projectile leaves the muzzle, the quieter the shot will be. This is how the Metrogun shotgun extension barrels work or why bloop tubes or reverse paradox tubes have noticeably lower reports.
A long barrel is, essentially, a sort of suppressor. Powerful projectiles can be quiet, out of long-barreled guns.
158-grain or 148-grain cast bullet loads in .38 Special or .357 cases, with 2.5 grains of a fast-burning powder such as Bullseye or Red Dot, are very quiet out of barrels 22 inches long or longer. Velocity is reported to be about 800 fps. Lubricated lead bullets are quieter than copper-jacketed bullets because copper jackets require more pressure to push them through the bore.
The loads do not require small-capacity cases. A correspondent tells me a 315-grain cast lead bullet, out of a 25-inch .416 barrel, propelled by 5-8 grains of Red Dot, is subjectively about as quiet as as an air rifle. Velocities vary between 700 and 900 fps.
There is a good discussion on these types of loads, from 2009, at go2gbo.com.
Here is an example:
Cat sneeze loads are pistol rounds fired in a rifle just under sonic speeds (700-800 fps) 3.5 grains of bullseye and a 140-200 gr semi wadcutter in my 357 handi rifle sounds like an air rifle , long barrel attenuates muzzle blast but still hits with SERIOUS authority, accurate 1-2 inch groups at 20-50 yards economical and quiet way to fill stewpot! Grin and pest control is serious fun!
Subsonic shotgun loads in long barrels are also relatively quiet. 1 1/8 ounces of shot propelled by 6-7 grains of Red Dot is subsonic and said to be effective to 20 yards with #8 pellets. If loaded with # 4 pellets, it should be effective out to 30-40 yards. The longer the barrel, the quieter it is.
CCI has had great success with its Quiet .22 loading. Perhaps a manufacturer will come out with a Quiet 12 gauge load for use on pests when one wishes not to disturb the neighbors.
Light loads can be very effective on pests at close range. A 158 grain .38 at 800 fps, from a rifle, is a practical load to 75 yards, more if you are a good judge of distance. Large animals can be taken with proper shot placement.
Notice: An important consideration with low-velocity, low-pressure loads, is to be sure the projectile leaves the barrel. Always check if you are uncertain.
About Dean Weingarten:
Dean Weingarten has been a peace officer, a military officer, was on the University of Wisconsin Pistol Team for four years, and was first certified to teach firearms safety in 1973. He taught the Arizona concealed carry course for fifteen years until the goal of Constitutional Carry was attained. He has degrees in meteorology and mining engineering, and retired from the Department of Defense after a 30 year career in Army Research, Development, Testing, and Evaluation.