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Thread: What makes the noise?

  1. #1

    What makes the noise?

    Hi all

    just trying to understand what makes the noise, is it the speed or grain of bullet or a mixture of both? So for example if you downloaded a 55grain 223 to the same speed as a 17 grain hmr would the noise be the same or louder due to the fact it is heavier?

  2. #2
    I think you might find its two things.....something to do with x grains of propellant ignited in a steel pressure chamber and a small lump of lead passing Mach 1

    The noise is a combination of hot gases escaping the end of your barrel (the "boom") and the projectile breaking the sound barrier as it screams away (the "crack")

    Yes....theoretically, less propellant would give less "noise" as there is "less" of an explosion!

  3. #3
    Thanks and i guess Less explosion also means slower bullet so less crack too?

  4. #4
    I always though the "crack" part of the noise was the bullet going supersonic, hence why .22 subs are so quiet

  5. #5
    Quote Originally Posted by Ah8217bris View Post
    Thanks and i guess Less explosion also means slower bullet so less crack too?
    A slower bullet might mean it goes subsonic then NO crack at all, but if it goes supersonic, there will always be a crack. There's either a crack or not. Not varying loudness.

  6. #6
    SD Regular Greener Jim's Avatar
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    Jul 2014
    Yorkshireman in Darkest Cornwall
    The crack is unavoidable unless subsonic. The associated bang is down to how much powder is being burnt, less powder, less bang.
    Thats not all though, long barrels are quieter than short barrels and small bores are quieter moderated than big bores moderated.

    So to answer your question, no the .223 would still be marginally louder even if all other variables (powder charge, barrel length etc) were the same.

    The weight of the projectile, as far as I am aware, has no bearing on the noise. The diameter does though as mentioned above.

  7. #7
    Thanks Jim, that makes a lot of sense. I guess linked to this are there any powders you can use which are slightly quieter as you need to use less of them? Or does type of powder make no difference just the quantity to the boom?

  8. #8
    Thanks Walshie, i thought there were varying degrees of crack (so to speak!) but thanks for clarifying

  9. #9
    Muzzle blast levels vary according to the quantity, velocity and release pressure of the escaping propellant gasses. As such, the key factors are propellant charge weight, maximum pressure achieved in trhe barrel, calibre and barrel length. Fire a snubby .44 Magnum revolver with a factory load and you get a lot of noise; a .22 Hornet from a 30-inch rifle barrel and it'll be far less as the gas muzzle pressure is very low as is the weight / quantity of gas.

    The supersonic 'crack' comes from the projectile forming a shockwave in the air ahead of its tip. The faster and bigger the item in flight, the greater the shock-wave size and hence extent of its spread throughout the surrounding atmosphere. Have a supersonic 50gn .22 cal bullet pass you a few hundred feet overhead and you might not even hear its passage. Repeat the exercise with a modern warplane at the same speed and it'll smash every window in your house and could even cause structural damage such is the size and power of the shockwave. When marking targets on a rifle range, it's quite easy to distinguish .22 and 6mm bullets hitting the target from 7mm and .30s, their 'crack' being noticeably quieter. Some rifle clubs now insist on hearing protection in the butts such is the noise from heavier supersonic bullets passing a few feet above the head.

  10. #10
    Basically three things affecting firing noise.

    Precursor air column: This is the output created by the discharge of the air column that lies in the barrel in front of the cartridge. This air becomes compressed on firing and at release at the muzzle creates a shock wave.

    Projectile exit: Again caused by the disruption of air, in this case by the bullet, even at subsonic velocities. Once the bullet exits the muzzle it pushes air out of the way as it moves down range. After the bullet has passed the air will fill the void left by its passage. Big bullets move more air than smaller ones and air noise causes sound to propogate from the bullets passage. If the bullet is large enough and above supersonic speed, the collapsing vorticies from the bullet's base will create a whistle down range. At subsonic speeds the wistle disappears but the noise of the bullet's flight gives a clear indicator of the direction that the bullet was fired from. Again bigger bullets make bigger noises.

    Propellant gas discharge: This is almost always supersonic and is both hot and compressed creating turbulence (noise) when it collides with the air outside of the muzzle.


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