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In order to understand this, there are a few things about the functioning of the AR that have to be defined, I am away from most of my notes and stuff, so most of the figures given are from memory... but should be pretty close. For this description, the standard rifle gas system is with the gas port located at 13.0" and having a 20" barrel... the standard carbine system is with the port at 7.5" and a barrel of 14.5"
The pressures at the gas ports are: 13.5K for the rifle and 26K for the carbine -- or twice as much.
The dwell time (the time that the gas system is charged with high pressure) is determined by the amount of barrel after the gas port. These are nearly identical between the rifle and the carbine.
Pressure from the port is regulated only by the size of the gas port and the diameter of the barrel.
These two factors determine the internal bolt pressure, the maximum pressure that is obtained in the bolt carrier/piston combination -- for the rifle this pressure is about 1000psi and for the carbine it is over 1500psi, half again as much.
When the rifle is fired, primer shot sets the bullet forward until it contacts the rifling, at this point the powder charge detonates and sets the shell case fully back, binds the action and start to propel the bullet. The bullet jumps slightly again and is etched by the rifling... it stops again very briefly as the pressures build to a point for the bullet to overcome the mechanical advantage of the rifling twist and the bullet starts to spin, at this point the chamber pressure is at max, 50K plus (there are some that believe there is another, third stop the bullet makes and some testing suggest this may be true).
As the chamber pressures start to climb, the brass case expands and becomes plastic, this is essential to seal the case in the chamber -- the correct term for this is Obturation, when the case is obturated and sealed, it is stuck in the chamber, practically welded in really.
The Lock Time, or the time that the action remains locked with no attempt to start unlocking is very important... on the rifle, the lock time is about 550 microseconds, the lock time for the carbine is about 375 microseconds -- this may not seem like much, but it is much shorter of a time, also keep mind that the chamber pressures are twice as high in the carbine when the unlocking starts.
What does all of this mean? When the carbine is fired, the system attempts to unlock earlier than intended and while the case is still fully obturated... this results in the action bind delaying the unlocking and stressing the system. As the 5.56N is not drastically tapered, "squirting" is not a big problem in most guns. When the internal bolt pressures finally unlock the bolt, the velocity of the reward movement in the carbine is much higher than what the rifle was designed for, it also must start extraction of the obturated case... as you know, the AR does not have any sort of initial extraction, perhaps the single biggest shortcoming of the design. This has been known to cause ripped case heads...
At this point, as the bolt starts to unlock, it is rotated to unlock...here there is a phenomenon called extractor lift where the extractor lifts off the rim of the cartridge case -- some argue that the pressure of the extracted cartridge case keeps the case head against the bolt face, but the fact is that the extractor does float and the contact with the case rim becomes "soft". For this reason, it is much more likely that the extractor will simply pop off, rather than actually rip the case.
Balanced extractors and different designs have been developed (LMT), but the best solution to date has been stronger extractor springs and spring buffers. That about covers the FTE issues...
Back to bolt velocity. The high speed of the bolt has a couple of other detrimental effects, one of the most common is that the bolt is cycled so fast that as it returns to battery, it actually has enough force to "bounce" off of the barrel extension when closing and locking... this bounce back is very small, but can be enough to cause the weapon not to fire... this "bolt bounce" is pretty well known.
One other problem is that the bolt can cycle so fast the magazine spring can not keep up with it and the round stack is not properly aligned and forced back into place before the bolt returns to batter -- therefore there is no new cartridge picked up and the bolt closes on an empty chamber, this is what some call "ghost loading", or bolt-over-base jams... this is far worse in full auto fire as the bolt does actually move faster in full auto than semi auto; this is due to the fact that the top cartridge in the magazine does not apply force to the bottom to the bolt causing drag.
The common solution to this issue is to use a stronger recoil spring and a heavier buffer... this works, but is treating the symptom, not the problem.
PigTail and expansion chamber gas tubes attempt to fool the rifle into thinking that the gas port is, located further away that it really is, but they are not as good of a solution as actually moving the gas port out...
