There are 3 sources of sound produced by firearms:
1.Muzzle gas and sound escaping the chamber through the barrel. Usually second loudest sound
2.The sound of the bullet breaking the sound barrier. Usually loudest sound. (lower when the air is a colder temperature, important for selecting loadings).
These two cover 90->99 percent of all firearm sound
3.Action of the gun.
A semi-automatic or automatic firearm tends to produce noise as it cycles the actiob, stripping a spent case out, inserting a new bullet, firing pin hitting the primer to fire the bullet.
Now how about we make a silent firearm?
Firstly, a suppressor slows down the expansion of gas from the barrel of the gun.
Secondly, a subsonic bullet, heavier, sometimes less powder loaded, is made to travel lower than the speed of sound.
Now already we are quiet enough to fire undetected from a distance, depending on the sound of the action, we are quieter than a whisper or quieter than your friend yelling to you across the room.
If we want to make it trully silent? Bolt action. A smooth bolt action will be undetectable to the human ear. A suppressed bolt action .22 rifle will be so quiet that you won't know it fired until you see the bullet hole. You can fire next to someone's ear and they will have no idea, you can sit down and clear the room you are in with well placed aim and the only sound will be bodies hitting the floor. Quieter than a knife. Trully silent. Even semi-automatic .22 rifles firing subs can be effectively silent. May take some tweaking with swapping out the bolt or trigger group depending on the rifle to get complete silence. Ruger 22 takedown is an excellent example. The implication is that you can have as an American, as in many other countries, a maneuverable, lightweight, semi-automatic rifle that can be broken down and reassembled in 5 seconds to fit in a small bag or backpack or in a coat flask pocket which is completely silent and can reliably kill within 200-300 metres with good aim and and decent optics, entirely legal and unregulated except for the supressor, which is a form and a year long wait and a tax stamp or otherwise extremely easy to construct and conceal from a wide variety of materials in temporary or permanent constructions. A water bottle and duct tape will work great for one or two shots as shown in Chechnia.
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