Originally Posted by
Failure2Stop
I can't get too specific, as there are proprietary matters, however, I can disclose that we use high-speed video and pressure sensors that are accurate to silly small measurements, and have really smart engineer guys that translate that data for knuckle-draggers like me. I was corrected in that speed of sound was the local speed of sound, not the ambient speed of sound. Mea culpa.
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?
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) However, when contained in small vessels (cartridge cases & bores) the pressures generated can exceed the material strength of the barrel / firearm. 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.
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.
Originally Posted by
Failure2Stop
The collected physical data shows that the bolt carrier begins rearward movement in about twice the time it takes for the projectile to travel from the chamber to the muzzle.
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. 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. 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.
ETA - on the flip side and as an example of momentum - look at an M1 Garand - the port is an inch from the muzzle - that bullet barely even travels one caliber before it clears the barrel - so the propellant gas gets a RHCH nanosecond to impart a punch to the oprod - its all momentum baby!
Originally Posted by
Failure2Stop
As the carrier begins its rearward movement, it travels approximately 0.075" before the cam-pin hits the first angle of the cam path, which gives additional delay to unlocking.
I absolutely agree and understand that. This is engineered into the design This delay (aka dwell) is required to allow chamber pressures to subside in order for extraction to occur. Momentum is force multiplier! This dwell is also part of the inherent accruacy of this system. If a shooter can exhibit proper follow through with an AR type rifle, there is no mechanical disadvantage to the system other than the slow lock time, but that is up front, prior to ignition. Unfortunately this isn't the case with an M4 type system - the early bolt unlocking and high rotational speed of the bolt actually begin when there is still significant pressure in the chamber.
As a qualifier, I am a bit reluctant to say that this isn't merely a hobby. I don't like to get into resume' competitions - but I offer this as a bit of a qualification to the information presented, this isn't stuff I pulled out of a comic book while fondling myself in my mothers basement - I work for one of the premier DOD R&D/S&T labs in the country and my section deals with ballistics, blast protection, terminal performance, body armor and individual protection and individual equipment as well as ballistic threat assessments. We live it, breathe it and eat it everyday. Hopefully the information will provoke rational thought and discussion of these wonderful little rifles.
Last edited by opsoff1; 08-13-15 at 19:35.
opsoff
"I'd rather go down the river with seven studs than with a hundred shitheads"- Colonel Charlie Beckwith
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