Here's what Armalite has to say. The reason they move the gas port 2" forward.
December 6, 2001
Technical Note 48, Effects of M16 Rifle and Carbine Barrel Design and Heat on Reliability
The M4 Carbine has developed a reputation for poor reliability. The excessive malfunction rate of the M4 Carbine is due to physical imbalances in the mechanism itself, exacerbated by heat. Analysis of the problem provides a good understanding of the carrier group and barrel of the rifle, and the function of the cartridge case.
Relation of carrier group design, barrel length, gas port location, and propellant gas pressure. The location of the gas port, length of barrel remaining beyond the port, and characteristics of the propellant gasses determine the amount of energy provided to power the action of the M-16 series rifle. The heart of the M-16 operating system, the carrier group, was designed to function well with the original 20 inch long barrel of that rifle.
A change in the cartridge (bullet weight or powder), length of the barrel, or location of the gas port along the barrel can substantially change the gas pulse that enters the carrier group and drives the rifle action. Short versions of the M16 (including the M4) suffer from relocation of the gas port and changes in barrel length.
The carbine gas port is located closer to the chamber than the gas port of the rifle, 7.5 inches ahead of the case mouth instead of the 13 inch distance of the rifle. The gas pulse enters the carrier group earlier than with the rifle length barrel because it travels a shorter distance. The propellant gas pressure at that location is double that of the rifle, at 26,000 psi vs. 13,000 psi.
Both the early pressurization of the carrier and the higher pressure at the closer port tend to cause the carbine to extract earlier and faster than the rifle, at the same time that pressure within the cartridge case is higher than in the rifle. The cartridge case has less time to shrink away from the chamber wall before extraction begins, and resistance to rearward movement is high because the case wall adheres to the chamber wall, especially when the weapon is hot. This stresses the extractor and cartridge case.
The faster movement of the carrier group also creates an interesting and largely unknown outward movement of the extractor. Because the extractor pin is mounted toward the rear of the extractor, most of the extractor's mass is ahead of the pin. The bolt turns to the unlocked position so fast that centrifugal forces tend to drive the unbalanced extractor open at the front end, and it disengages the extractor. The faster movement of the M4 carrier increases this loss of control of the cartridge and the extractor tends to stay open longer than it does in the rifle. This can decrease extractor efficiency.
Effect of heat on rifle function. Heat increases the carbine problems listed above. Understanding the effect of heat requires a firm understanding of the purpose and action of a component of the rifle system that is often overlooked: the cartridge case. The cartridge case is a highly sophisticated component that performs a number of functions
1. It holds components (bullet, primer, powder) together precisely.
2. It engages key surfaces of the magazine and rifle to transport these components into the chamber.
3. Upon firing, it expands into intimate contact with the chamber wall to seal high pressure gas in the barrel.
4. It contracts from the chamber wall when pressures lower to an acceptable level and when some of the heat within the case is transferred to the chamber wall.
5. It transports heat out of the weapon when extracted.
When the weapon overheats, it is harder for the cartridge case to transfer heat to the hot chamber wall and shrink away from it. Cartridge case adhesion to the chamber wall results in increased resistance to extraction, increasing stresses on the extractor.
Effect of heat on the barrel.
In addition to increasing the malfunction rate, excess heat weakens the material of the barrel.
The barrel of the M4 carbine is made of chrome-molybdenum-vanadium steel, and is chrome lined. It is an extremely high quality grade of steel capable of long service. This steel tolerates high temperature well. At a temperature of approximately 1100 degrees, however, the structure of this alloy undergoes a permanent transformation that substantially, and permanently, alters it. The steel becomes prone to rupture under high pressure. It may not fail at the time of overheating, but instead may fail at a later date and far lower temperature. This confuses the user as to the reason for failure
Options for improving basic carbine function.
1. Reducing the diameter of the port to reduce gas flow to the carrier. This can render the mechanism somewhat more prone to powering problems than the larger gas port of the rifle.
2. Providing an expansion chamber to buffer the gas flow. Such chambers can become clogged with powder residue, reducing the buffer effect. The flash hider of the Vietnam-era XM177 Carbine provided a degree of buffer effect.
3. Shifting the gas port. ArmaLite has shifted its carbines gas port two inches forward of the M4 Carbine’s gas port location. Since the commercial minimum barrel length is set by law, this change serves reduce the gas pressure, delay its transmission to the carrier group, and reduces the length of barrel (and thus gas pulse duration) beyond the gas port. This change is easy in a commercial setting but has logistics implications for the military.
4. Decreasing the tendency of the extractor to open by increasing extractor spring strength, adding material to the extractor to reduce the tendency to open, or adding various elastomer forms to the extractor to reinforce the spring.