Here's Homer Powley's eleventh Technical Memo, "Muzzle Blast," from December 1963 -- the whole thing, more or less as it will appear in the book-form collection of Homer's TMs when I get it all put-together and publish it, as he requested:
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<br>You probably don�t know where most of the noise of a gun comes from. You have read about gases rushing out of the muzzle rapidly and then, with high-velocity guns, the formation of a shock wave by the bullet; these two things make noise. If you don�t know what a vacuum is, you might believe that there is a vacuum in the barrel after the gases are gone, so the collapse of the vacuum makes the noise; don�t believe this one.
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<br>In addition to noise from gas and bullet departure, there is a muzzle blast that makes most of the noise, and you can many times see it because of the flame. With large rifles, the noise is not a sharp, annoying crack but rather a slower �boom!� Just a few hundred yards from the gun, this is about all you hear unless the bullet passes very close to you, when you also hear the crack of the shock wave before you hear the gun. From a distance, the boom of a large rifle seems to lose direction. This is the reason that near settled communities, large rifles, unless very close, are not as annoying as small rifles, handguns, and shotguns.
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<br>Most practical rifle shooters know about muzzle blast, and a few of them know that unburned powder can be found on the ground a short distance from the muzzle. These few have then jumped to the conclusion that since not all of the powder has burned inside the barrel, it is burning outside the muzzle, and this is where the blast comes from. Don�t believe this one either.
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<br>With normal loads, only about 99 percent of the powder burns inside the barrel. The rest goes out the muzzle in the form of splinters � you can tell when you find them. All of the powder that is going to burn has done so at the place of peak pressure, which is only a few inches in front of the chamber. Some experimenters have used small cannon powders in rifles where only a low pressure (and velocity) develops, and unburned powder in the form of thin-walled tubes can be found, not only spitting from the muzzle but also trailed down the bore and left inside the case. In either case, powder burning in front of the muzzle can make only very slight noise.
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<br>When gunpowder burns under pressure, the solid is converted to a mixture of gases including nitrogen, steam, carbon dioxide, carbon monoxide, hydrogen, methane, free carbon (and from black powder, hydrogen sulfide). All of these � except nitrogen, steam, and carbon dioxide � undergo combustion in air. The relative amounts formed depend upon the kind of powder, the temperature and pressure when formed, the presence of catalysts, and the variations during expansion down the bore. They react with each other to change the composition. For example, carbon dioxide and hydrogen combine to make carbon monoxide and steam � this is known as �the water-gas shift reaction� and is reversible.
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<br>When these gases come out of the muzzle at high temperature and pressure, they ignite in the air to cause a secondary explosion by combustion and with the formation of a ball of fire. Most powders contain salts of some kind to reduce the blast and flash. An attempt is made to control the carbon monoxide if a gun is to be used in a closed space. Muzzle brakes or flash-hiders tend to spread the gases into cooler air with dilution and so reduce the effect.
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<br>So now you know.
