Is the RRA CAR or Mid-length really that bad?

[BufordTJustice]

The alteration from the original ~10" gas system length to 11" and eventually to 14.5" was done by Colt and not by Stoner (who rejected the alterations to the platform altogether). You'll find that the final design of the M4's gas system (incorporating the gas tube length and the dwell time/length of barrel after the gas port) was determined by the Army's requirement to fit a standard M16 bayonet. It had little, if anything, to do with the engineering of a proper gas system for the platform.

http://en.wikipedia.org/wiki/CAR-15#Colt_Model_653_M16A1_Carbine

"In the early 1970s, Colt began development of an M16A1 carbine with a 14.5-inch long barrel. The 14.5-inch length was compatible with the existing carbine-length gas system and allowed for the mounting of a standard M16 bayonet."

There is a reason that it took Colt from 1965 to 1984 to get the CAR gas system to work properly. After being handed a fully functional rifle design, it took them nearly 2 decades to get the shorter gas system to work.

[MistWolf]

Buford, please go back and re-read what I posted. I don't bash the middy. I like the middy (and bought one) but my reasons are personal rather than technical. I do not claim the carbine is superior. If the middy is easier on bolts, let's see the test that support the claims. I am a technician. I think like a technician. If I tell someone this is better than that, I must have the data to back up those claims. I have seen no data supporting the claims of middy, or carbine, superiority.

Gas is tapped in a carbine system where pressure is higher, but gas port size controls the time it takes to pressurize the system. No one has ever shown what the pressure of the gases reaching the BCG is. No one has shown what velocity the gases travel the gas tube. No one has shown how much heat the gases have as they enter the expansion chamber. Without data, we are guessing and often the guesses ignore the laws governing thermodynamics, Bernoulli's Principal or how compressed gases behave at supersonic speeds.

Until pertinent data is collected and analyzed or proper mathematical calculations are made, we cannot know what is happening. People are still spouting the nonsense that an extra two inches of barrel length causes higher bolt speeds because of the extra time the bullet is in the barrel despite the fact the difference in time is less than half of one millionth of a second and the bullet is long gone before the gases even pressurize the BCG!

[BufordTJustice]

Buford, please go back and re-read what I posted. I don't bash the middy. I like the middy (and bought one) but my reasons are personal rather than technical. I do not claim the carbine is superior. If the middy is easier on bolts, let's see the test that support the claims. I am a technician. I think like a technician. If I tell someone this is better than that, I must have the data to back up those claims. I have seen no data supporting the claims of middy, or carbine, superiority.

Gas is tapped in a carbine system where pressure is higher, but gas port size controls the time it takes to pressurize the system. No one has ever shown what the pressure of the gases reaching the BCG is. No one has shown what velocity the gases travel the gas tube. No one has shown how much heat the gases have as they enter the expansion chamber. Without data, we are guessing and often the guesses ignore the laws governing thermodynamics, Bernoulli's Principal or how compressed gases behave at supersonic speeds.

Until pertinent data is collected and analyzed or proper mathematical calculations are made, we cannot know what is happening. People are still spouting the nonsense that an extra two inches of barrel length causes higher bolt speeds because of the extra time the bullet is in the barrel despite the fact the difference in time is less than half of one millionth of a second and the bullet is long gone before the gases even pressurize the BCG!

Mist, using the same principles of the original design, we can make factual inferences about alterations to it.

Using the rifle system as a reference point, the CAR exhibits higher port pressure than the midlength.

Using the rifle system as a reference point, the CAR system generates earlier unlocking of the bolt than the midlength.

Using the rifle system as a reference point, the CAR system must have a higher cyclic rate than both the midlength and the rifle.

Using the rifle system as a reference point, a higher cyclic rate represents additional kinetic energy being deposited into the reciprocating bolt carrier group. Seeing that the rifle system has been repeatedly shown in tests by the Army to have a longer mean rounds between failure number than the M4 carbine, one can infer that the gas system (and the buffer system) on the rifle are superior to the CAR gas system employed in the M4.

I don't need hard numbers to know that on the midlength system the bolt unlocking is slower/occurs later than in the CAR system, the rearward acceleration of the BCG upon extraction will be less than in the CAR system, the shear forces placed on the bolt lugs during extraction will be less on the midlength system than in the CAR system, and the dwell time will be shorter (meaning a smaller overall gas volume) in the middy than in the CAR.

I don't need to have exact numbers. Basic mathematic principles dictate that lower pressures and slower operating speeds, which are more in-line with the original design, can ONLY have positive effects in the function of the weapon. I lack the education and the tools to measure and generate true empirical data on HOW MUCH of an improvement there is. However, I don't need these measurements to know that THERE IS AN IMPROVEMENT when moving to the middy gas system over the CAR.

Getting closer to the original design (which has demonstrated superior reliability to the CAR system) is going to guarantee improvements in reliability and parts life. How much? I don't know exactly how much. But more mean rounds between failure, smoother recoil impulse, and longer overall parts life is a legit benefit that comes currently at no extra cost (hardly anybody is charging premiums for middy over CAR gas guns).

Additional parts life, longer service intervals, more "headroom" for parts failure (if service intervals remain the same), longer bolt life, more reliable feeding and extraction, and smoother recoil impulse (if only slightly)....all for NO extra money out of pocket....show me the downside to that. I don't see one.

As far as measuring gas speeds in-bore and in the gas tube...I'm sure that the amount of time required for the bullet and gas column to traverse a 13" gas system (rifle length) would sound equally tiny to an untrained observer. We're drawing finer distinctions here

Most people don't care about gas ports. Most people would not think of the difference between the ~.061 size port on a Colt 6920 and the .079 sized gas port on a Bushmaster that I recently measured as anything but trivial in nature. But that tiny difference makes the bushmaster feel like it is chambered in 6.8 SPC vice the Colt's (obvious) 5.56x45mm chambering using the same ammo. This is a "game of inches" in football terms.

[MistWolf]

Mist, using the same principles of the original design, we can make factual inferences about alterations to it...

Yet the higher port pressure does not mean the carbine length will have a higher cyclic rate. How fast the system pressurizes is controlled by port size. Does the .mil have a different cyclic rate specification for the M4 than they do for the M16?

The AR gas system is self regulating. Once the BCG reaches proper pressure, it moves, separating the gas key from the gas tube. This stops the flow of gas into the BCG. Assuming the gas port is the correct size and the ammo generates the right pressure curve, it will give us the right cyclic rate. (This is why the larger port size messes up the system, as you so rightly point out. It lets the BCG charge too fast before it begins moving, resulting in too high a cyclic rate.)

The rifle length uses a larger port size which allows the system to pressurize faster. Since the system pressurizes faster, combined with a longer distance to travel, isn't it possible that the cyclic rate of a rifle length would be the same as the carbine or the middy?

Where does the extra shear force on the lugs come from? It can't be from simply yanking the bolt open at a higher cyclic rate so long as the cam slot in the carrier is cut correctly. As the carrier moves rearward, the cam slot only turns the bolt. The bolt isn't yanked back until the bolt is done rotating and the lugs are clear inside the barrel extension. It cannot be from extracting the case before blow down is complete because that should only place stress on the extractor, unless extraction causes distortion to the bolt. (If you're talking about extra forces on the cam pin hole on the bolt, then I misunderstood what you were talking about and apologize.)

While we can draw conclusions from inferences, we still need to ask the right questions and apply the correct principles. Also, we must keep in mind that until we prove the conclusions, they must remain suppositions

[BufordTJustice]

Yet the higher port pressure does not mean the carbine length will have a higher cyclic rate. How fast the system pressurizes is controlled by port size. Does the .mil have a different cyclic rate specification for the M4 than they do for the M16?

Not that I know of, but the cyclic rates do vary widely with the rifles commonly between 650-750 rpm and the M4's reaching toward 900 unsuppressed. I've never seen a rifle gas gun cycle at anywhere near 900 rpm, though I'm sure it's possible to engineer one that does.

The AR gas system is self regulating. Once the BCG reaches proper pressure, it moves, separating the gas key from the gas tube. This stops the flow of gas into the BCG. Assuming the gas port is the correct size and the ammo generates the right pressure curve, it will give us the right cylic rate

Yet, again, the cyclic rates differ by about 200 rpm between the rifle gas system and the CAR system.

The rifle length uses a larger port size which allows the system to pressurize faster. Since the system pressurizes faster, combined with a longer distance to travel, isn't it possible that the cyclic rate of a rifle length would be the same as the carbine or the middy?

No, because the dwell time is far less in proportion to the volume of gas being sent through the tube. In addition, the gas being tapped at a rifle gas port is going to be at a lower pressure than at the CAR position.

Where does the extra shear force on the lugs come from? It can't be from simply yanking the bolt open at a higher cyclic rate so long as the cam slot in the carrier is cut correctly. As the carrier moves rearward, the cam slot only turns the bolt. The bolt isn't yanked back until the bolt is done rotating and the lugs are clear inside the barrel extension. It cannot be from extracting the case before blow down is complete because that should only place stress on the extractor, unless extraction causes distortion to the bolt. (If you're talking about extra forces on the cam pin hole on the bolt, then I misunderstood what you were talking about and apologize.)

The extra force on the bolt lugs comes from extraction beginning before case obturation has subsided. The CAR system on NATO ammo is beginning extraction BEFORE the case has completely "shrunk" down to a normal size. Any friction generated by this "stiction" as it has been called is going to be transferred to the bolt. To the lugs in the form of rearward pressure from the case head and also to the extractor which is attempting to rotate the just-fired case while it is still sticking to the chamber walls. Anything that minimizes this early unlocking is a good thing.

While we can draw conclusions from inferences, we still need to ask the right questions and apply the correct principles. Also, we must keep in mind that until we prove the conclusions, they must remain suppositions

I disagree with the last paragraph and here's why; these aren't conclusions based on inferences. They are estimates based on known operating principles (the rifle gas system). Any modification that attempts to bring the platform more in line with the rifle system will display more of the rifle system's attributes. I agree that the right questions need to be asked, but we already know the operating principles of the rifle system. It is as fully a known quantity as any other currently employed small arm on the planet. I disagree that my statement that "the midlength gas system is an improvement over the CAR length system" must remain a supposition. Maybe we will have to agree to disagree on that point.

.....

[MistWolf]

The rifle length uses a larger port size which allows the system to pressurize faster. Since the system pressurizes faster, combined with a longer distance to travel, isn't it possible that the cyclic rate of a rifle length would be the same as the carbine or the middy?

No, because the dwell time is far less in proportion to the volume of gas being sent through the tube. In addition, the gas being tapped at a rifle gas port is going to be at a lower pressure than at the CAR position.

The gas port regulates flow. The smaller port of the carbine system reduces the volume per second* the gases flow into the system. That means the volume flow of higher pressure gas (which is denser) will be about the same as the volume flow through the larger rifle port with it's lower pressure gas. This regulates cyclic rate. When the cyclic rate is kept within the specified range, stress on the BCG should be the same as the acceleration rate of the BCG will be about the same.

*Gas volume varies with pressure. One pound of air takes up less volume at 25,000 psi than it does at 15,000 psi. What the gas port of the AR really does is ensure the correct mass of gas at the right pressure is tapped into the system at the proper rate for the pressure being tapped.

I am not sure what you mean by the dwell time being less in proportion to the volume of the gas being sent through the tube.

I do think that the carbine system, due to tapping gases at a higher pressure point, is less forgiving, that the gas port diameter must be more precise for consistent operation. I also accept that my conclusions are suppositions until data can be collected and analyzed. However, my suppositions are based on the application of physics. That's why I'd like to see the math (which I admit to being wholly inadequate to apply) and/or actual collected data to analyze in order to test those conclusions

[Quiet-Matt]

I have a SBR that is built on a registered RRA lower. That lower is GTG and the gun is one reliable bastard. Why is it so reliable? Because I removed all the "crap" from that original RRA factory gun, and all that was left was a lower reciever and a trigger guard. The rest was replaced with quality components from DD, Colt, LMT, Centurion Arms, BCM, Troy, Magpul, and Vltor. You might notice that RRA is NOT on the list of manufacturers whose components I used to make the gun "reliable".

[djmorris]

RRA is over priced junk.

Terrible barrels, terrible quality control, terribly cheap materials being used, gimmicky snake oil accessories. They are junk.

[drparkwood]

RRA is over priced junk.

Terrible barrels, terrible quality control, terribly cheap materials being used, gimmicky snake oil accessories. They are junk.

Wow this was helpful insite.

transmitted from Apollo 11 on the dark side of the moon

[djmorris]

Wow this was helpful insite.

transmitted from Apollo 11 on the dark side of the moon

What more do you need to know? Everything thing I said is accurate and the information is readily available on this very forum, and many other places. They cut so many corners that they are still using commercial spec receiver extensions - that speaks volumes, IMO. Their main focus is the "fit and finish" because that's what joeblows who know nothing about firearms look for.