While I do agree that carbine systems have been made very reliable by the introduction of action parts which have improved their function, and our company has been at the forefront of the development of these parts, there is still an advantage to the midlength systems over the carbine.

In a carbine, the pressures and temperatures of the gas are very high at the gas port location.
This causes the eventual erosion of the gas port, which makes the hole larger and changes the pre-set metering of the gas into the action. As the port erodes, more gas is sent into the action, thus increasing action speeds, battering, rate-of-fire increase, extraction problems, bolt bounce, and double feeds. Or any mix of the above.

Even though the midlength gas port is only 2 inches or less more forward on the barrel, that is enough to reduce the pressures at the port by almost half what it is at the carbine port. As a result, this port erosion is significantly reduced, and less problems will occur because of that.

Now, I know that a nice heavy buffer(or an MGI RRB), along with an extractor improvement device, and an adjustable gas system can solve most of the problems that come along eventually with a carbine. But the midlength makes the problems become less likely, and any problems that may come along will be later in the life of the barrel, as opposed to sooner on the carbine.

Even though we at MGI are making what are probably the premier products for improving the function of an AR15 carbine system, we will soon be introducing our version of the midlength gas system for the AR15/M16.

The benefits of the system are undeniable, and we will introduce a system that has these benefits, and  also a way to get into midlength at a more reasonable cost than many of the existing systems, because of certain compatibilities with existing carbine parts like handguards/free-float tubes. Look for this system to become available before Labor Day.

It actually begins right away, but usually doesn't become noticeable to the average shooter until some problems with reliability begin to show themselves.

It would be much more easily noticed as a rate-of-fire increase with a full-auto gun, and the user would notice it running faster fairly easily.

It will typically start to make a difference around 500 rounds fired, and may not be noticed as a problem until maybe 2000 rounds. But, it's happening. Some people will never notice it, until they start having failures, because they may not be very tuned-in to the way the gun runs.

I can pretty much tell you that if you have a reasonable amount of rounds thru your carbine, especially if you have moved to a heavy buffer to improve reliability, then you have some port erosion happening in your barrel.

Since it happens gradually as you shoot, you don't notice it easily when shooting semi-auto.
Also, if you only shoot slow fire, and don't bump-fire or run some mags real fast alot, then it will happen slower, but it will still happen.

Chewbacca,
You are probably right that most users will not notice the gradual erosion of the gas port in their semi-auto AR15 carbines.

But, there are a massive number of threads on the forums regarding extraction issues, double-feeds, bolt bounce, and general reliability topics about carbines on the AR15 forums here.
Many people may not realize that this gas port issue is the root cause of many/most of these problems.

However, if you had an M16 and when you got it with a fresh carbine barrel it ran at 800rpm, and now it's running at 1000rpm, then you'd notice that pretty easily, because you'd be dumping mags alot faster and getting a hell of alot more muzzle rise to the point of uncontrollability.

Now, of course I agree that this may not be a serious concern to many semi-auto users unless it starts causing stoppages of the gun. And this may not happen for a long time on some guns.

There is enough dwell time in a 14.5" barrel with a midlength gas system.
It should not be any problem to use a midlength with that.
I don't think that I'd go any shorter than that with a midlength, though.

The simple fact is that the microsecond that the gas pressure loads up against the piston, it is having an equal and opposing effect on the FSB, and thus the barrel(because the barrel CAN flex, so it WILL). This effect is happening whether the piston is moving or not. In fact, it may be preferable to have the piston moving at the earliest possible moment to reduce these negative effects to a minumum. An angular force vector is being introduced, that is not axial with the bore.

The piston cannot be actuated after the barrel has lost gas pressure(because pressure energy is what moves it), so if it does wait to move until after the bullet has exited, it is using energy previously stored, which is in process of overcoming the mass of the metal parts in the action before entering kinetic motion.

The physical gas laws state that a pressurized gas will fill its container with equal pressure on all sides. This means that the same pressure that is placed against the piston, will also be placed against the opposing wall of the FSB. At a carbine gas port location, this pressure is approximately 26000psi. You can think of it as a little balloon in there. When you blow up the ballon hard enough, if the piston doesn't move, the FSB will, thus flexing the barrel.

After the piston chamber is loaded up and flexes the barrel in one direction, then the piston begins to move(or the bullet exits, dropping the pressure), changing the pressure in the piston chamber, and then the barrel flexes back as the piston moves away and pressure is reduced. Depending upon the damping characteristics of the barrel, there may be an undamped decay pattern of harmonics introduced at this point, which will gradually subside in amplitude over a short time.

I'll grant you that the movement may be extremely small.
However, any effect on the barrel during the time the bullet is in the barrel will affect the barrel harmonics and likely will affect accuracy in some way.

This is not necessarily an indictment of piston systems, but simply a physical description of what occurs during the firing cycle of a piston operated gas system.

I have not discussed the effects of the moving masses above the axis of the barrel at all, but that is another subject.

Every system has its pros and cons.

In these types of systems, it is reasonable to conclude that because of these characteristics, there will be an effect of the piston operation on the harmonics of the barrel during firing, thus affecting the accuracy level to some extent, however large or small it may be. And, its effects are not minimized even if the piston does not move until the bullet clears the barrel.  The effects will be there regardless.

John,
I haven't really claimed any magnitude of the effect in this case.
I only stated that it is the way it works.

How much effect is strictly related to the system in question, and its design, and parts interfaces.

The only thing that is certain is that whatever effect is happening at the barrel, its effect on POI is magnified significantly as the distance from the barrel to the target gets greater.

Regarding the head-to-head testing, I have a feeling that is going to happen at SCAR.

Quoted: The simple fact is that the microsecond that the gas pressure loads up against the piston, it is having an equal and opposing effect on the FSB, and thus the barrel(because the barrel CAN flex, so it WILL). This effect is happening whether the piston is moving or not. In fact, it may be preferable to have the piston moving at the earliest possible moment to reduce these negative effects to a minumum. An angular force vector is being introduced, that is not axial with the bore. The piston cannot be actuated after the barrel has lost gas pressure(because pressure energy is what moves it), so if it does wait to move until after the bullet has exited, it is using energy previously stored, which is in process of overcoming the mass of the metal parts in the action before entering kinetic motion. The physical gas laws state that a pressurized gas will fill its container with equal pressure on all sides. This means that the same pressure that is placed against the piston, will also be placed against the opposing wall of the FSB. At a carbine gas port location, this pressure is approximately 26000psi. You can think of it as a little balloon in there. When you blow up the ballon hard enough, if the piston doesn't move, the FSB will, thus flexing the barrel. After the piston chamber is loaded up and flexes the barrel in one direction, then the piston begins to move(or the bullet exits, dropping the pressure), changing the pressure in the piston chamber, and then the barrel flexes back as the piston moves away and pressure is reduced. Depending upon the damping characteristics of the barrel, there may be an undamped decay pattern of harmonics introduced at this point, which will gradually subside in amplitude over a short time. I'll grant you that the movement may be extremely small. However, any effect on the barrel during the time the bullet is in the barrel will affect the barrel harmonics and likely will affect accuracy in some way. This is not necessarily an indictment of piston systems, but simply a physical description of what occurs during the firing cycle of a piston operated gas system. I have not discussed the effects of the moving masses above the axis of the barrel at all, but that is another subject. Every system has its pros and cons. In these types of systems, it is reasonable to conclude that because of these characteristics, there will be an effect of the piston operation on the harmonics of the barrel during firing, thus affecting the accuracy level to some extent, however large or small it may be. And, its effects are not minimized even if the piston does not move until the bullet clears the barrel.  The effects will be there regardless.

Interesting theory but incorrect for obvious reasons.  

Obviously.

John: V. Busy, will expand in detail shortly.  

Correct.  The pressure comes from the gas in the barrel, everything else is a tube.  Gas ports are not chromed due to the difficulty in getting a uniform surface thickness and hence port size.  Gas tubes are stainless steel; the inside of the gas nozzle and piston are coated in a high temperature Teflon the base metal being stainless.  The inside of the carrier is hard chromed in both systems.