Alrighty, had to tap one of my engineers for the following response since I am not an engineer.
Some of the reply had to be edited to remove proprietary knowledge and design information.
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Originally Posted by
opsoff1
Again - no disrespect, but I have no idea what you are trying to say. Local speed of sound? Ambient speed of sound? Ambient is the immediate surrounding - so I am assuming you are referring to the pressure and velocity of the gas at the muzzle?
RD engineering analyst responding, I don’t want to hurt any feelings, my goal is to educate, but please realize I cannot give any proprietary knowledge.
The gas field in a gas system consists of constantly varying pressures, velocities, and temperatures, where every location and point of time has a different speed of sound.
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Basic chemistry and more specifically, thermodynamics as related to firearms and ammunition adhere to all the same laws of physics as does everything else in the world. With that in context, small arms propellants are classified as low order explosives. The most powerful of these low order explosives are these same smokeless powders. In low order explosives, the process of burning or "decomposition" is termed deflagration. This process produces heat, light and a subsonic pressure wave. (Much to our testosterone fueled delight) However, it is important to note that this is in an uncontained state - like pouring a pile of 4895 on the ground and sparking it up. The pressure wave is subsonic, meaning it does not produce sound waves in the infrasound (very low) or the ultra sound (very high) frequencies (neither of which is in the range of human hearing) - so all you hear is the fizzing of the burning. There is no blast. So, when uncontained - the pressure wave or sound wave presents itself as subsonic - it doesn't exceed the speed of sound (1126fps in dry air at 68 deg F)
Nothing wrong here other than sound emanating from this burn occurs at the speed of sound( not less than) and this has nothing to do with combustion in firearms.
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However, when contained in small vessels (cartridge cases & bores) the pressures generated can exceed the material strength of the barrel / firearm.
This would mean that the gun is exploding, but since we know what these pressures are we are able to design for them, and alternately a light finger press could break a small enough part, but this really isn't pertinent to the discussion.
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These extreme pressures and rapid expansion of the decomposing propellant is what we see exiting from the muzzle as the bullet clears the barrel. High pressure,supersonic sound waves - give us the muzzle blast and accompanying explosive like sound report. Measurements and calculations indicate that the velocity of these gases exiting from the bore (known as the velocity of the reaction front) hover around 5200 fps for a long gun.
The process of burning the powder and building the pressure to move the bullet down the bore is a Pressure/Volume/Temperature problem. Smokeless powder contains its own oxidizing agent - it can feed itself. More heat builds pressure, more pressure builds heat, and so on - the variable is the volume of the vessel which changes as the pressure moves the bullet down the bore.
The combustion is an internal energy equaling PVT problem.
Only at immediate muzzle exit do you have a transition to supersonic, based on rapid acceleration behind the initial shock wave as energy contained in this wave dissipates it slows to a simple expanding sound wave. The time scale on this event is about 10^-6s. The muzzle blast is a pressure release not an explosion, there is no additional energy being released.
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In your original response, you stated that the pressure wave moves at the speed of sound - this is reasonable statement for propellant that is burning in an uncontained state. However, everything changes in a closed vessel. Since we have all sorts of burn rates, single base, double base and even triple base powders, the velocity of the reaction front at the muzzle is accepted within the engineering and physics circles to be 4000 - 6000 fps. As stated above and earlier - the mean is accepted to be approximately 5200fps for a rifle. Once clear of the bore an in an uncontained state, this gas pressure front cools rapidly and dissipates.
Burn rate has nothing to do with gas velocity and again at the muzzle there is no continued explosion, only the rapid expansions of gas into an ambient environment.
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I had to really think about this - and this is understandable, but it is very misleading. Establishing a point of measurement from the chamber to compare carrier movement is unfair as there is nothing to move the carrier. Nothing should be moving other than the bullet in the bore until the bullet passes the gas port. Once this happens, the expanding gas enters the gas tube and expansion chamber in the carrier.
We have the capability to see the pressure and motion and link them to a common time accurate to 1/200,000 s. The combustion can be seen as well as time pressure response at gas port and beginning of the gas tube.
The misconception is that everything needs to occur the same way a balloon fills, flow has inertia and once gas has started it will continue. This will occur at a predictable velocity based on pressure expanding into free air with a substantial wall friction.
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In a full sized 20" AR/M16, the port is 6.825" from the muzzle. This allows for a pressurization of the bore, chamber, gas tube and carrier for only the time that the bullet is between the port and the muzzle.
This is another gun lore misconception, as said below gas has momentum and does not need to have a sealed system to work. Using this example, projectile in bore time is 2.29*10^-4. Gas travel time in gas tube is substantially greater.
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That is a very very short amount of time. I have seen data that indicatates it is south of 1 millisecond. As soon as the bullet clears the muzzle - the pressure drops. What allows for the whole mechanism to work is the momentum of the gas "charge". This is where the port tuning becomes so important. Port location dictates the pressure available at the port. (13,000 psi for an M16, 26,000 psi for an M4) Port diameter dictates the volume of the gas that gets sent down the tube. In a crude analogy for port diameters, if you have a baseball traveling at 1000 fps and a bowling ball traveling at 1000 fps - which is going to do more work? Conversely, when we attach a can to the muzzle - we are in effect lengthening the bore and this pressurizes the system longer and cause all sorts of issues, not the least of which is hard recoil. Again - I point to the M4 problems - all directly related to the gas port location change and the accompanying spike in port pressures.
This is actually not the case, the analogy is much closer to stepping on a garden hose and flow decreasing.
Ok, me back:
The net of the discussion is this: original claim that "ARs want to jump, bounce and twist when you let a shot go, as the carrier starts to begin its cycle before the bullet exits the bore". Physical evidence proves that this is not the case unless intentionally producing a system that does so.