Quoted:
Uhh, Colt and Bushy don't make mid length uppers.

Quoted:
need more coffee

Technical Note 48, The Effects of Barrel Design and Heat on Reliability





Background:  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 requires a good understanding of the carrier group and barrel of the rifle, and the functions of the cartridge case.

Facts:

Relation of carrier group design, barrel length, gas port location, and propellant gas pressure. The distance from the chamber to the gas port, the length of barrel beyond the port, and the pressure of the propellant gasses determine the amount of energy provided to 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), the length of the barrel, or the location of the gas port along the barrel can substantially change the pulse of gas 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 instead of the 13 inch distance on the rifle.  The gas pulse therefore enters the gas tube sooner and reaches the carrier group earlier than it does in the rifle length barrel.  In addition to reaching the carrier sooner, it reaches it at higher pressure.  The gas pressure at the carbine’s gas port is double that of the rifle:  26,000 psi vs. 13,000 pounds per square inch.    

The early pressurization of the carrier causes the carbine to begin to extract earlier than the rifle does.  At the same time, the gas in the carbine’s carrier is of higher pressure than it is in the rifle, and it forces the carrier to move the rear at a higher velocity than it moves in the rifle.  Because of the earlier extraction, the cartridge case has less time to transfer heat to the chamber wall and shrink away from it before extraction begins.  The cartridge case has a tendency to stick to the chamber wall, and resistance to rearward movement is high.

When the bolt, drawn rearward by the high velocity bolt carrier, tries to pull this stuck case to the rear, both the extractor and cartridge case are heavily stressed.  The resistance can cause the whole mechanism to become sluggish or stop, or to cause early failure of the extractor or bolt.

As discussed above, the distance from the chamber to the gas port is important.  So too is the length of the barrel past the gas port.  

That’s because the bullet serves as a plug to keep the gas pressure trapped in the barrel so that some of it can pass into the gas tube and back to the carrier.  If the length of barrel beyond the gas port is too short, so is the “dwell” of the plug in the barrel.  The gas pulse supplied to the carrier can be too short to deliver all of the energy that the carrier group needs.  Too long a section of barrel beyond the gas port can cause too long a gas pulse.

Carbines with 11.5 inch long barrels have a very short segment of barrel beyond the gas port and the gas pulse is thus shorter than the carrier group requires.  This problem combines with the carbine problems already described, and the reliability of carbines with 11.5 inch long barrels is somewhat poorer than carbines with longer barrels.  Efforts to adjust for the short barrel by enlarging the gas port produce a firearm that is extremely sensitive to differences in ammunition.  Efforts to correct the problem by using different springs or buffers or by changing the volume of the gas used are only partially successful.

The faster movement of the carrier group in carbines also creates an interesting and largely unknown problem with the extractor.  Most of the extractor’s mass is ahead of the extractor pin.  The bolt turns to the unlocked position so fast that centrifugal forces tend to cause the front end of extractor to fly open, and it disengages from the cartridge .  The faster movement of the M4 bolt increases this centrifugal force and the extractor tends to stay open longer than it does in the rifle.  This tends to decrease extractor efficiency and increase extraction trouble.

Correcting basic carbine weaknesses:

1.  Reduce the diameter of the gas port to reduce high pressure 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.  Provide an expansion chamber to buffer the gas flow and/or store gas to power the carrier.  Such chambers tend to cool the gasses and can become clogged with powder residue over time, reducing the buffer effect.  The flash hider of the Vietnam-era XM177 Carbine provided a buffer effect and stored gas.

3.  Shift the gas port.  ArmaLite® has shifted its carbines gas port two inches forward compared to the M4 Carbine.  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.  Decrease the tendency of the extractor to open by increasing extractor spring strength, adding material to rear of the extractor to reduce the tendency to open, or adding various elastomer forms to the extractor to reinforce the spring.