Carbine Gas System?

[G11354]

Been reading articles covering AR-15 gas system lengths. Seems that the carbine length system is intended for 14.5" barrels and tends to be rough on the gun with 16" variety.

Are there any benefits to having a carbine system with a 16" barrel and are the claims above exaggerated?

 

[Malamute]

I think its somewhat picking nits, though the port pressure of a carbine is about double what it is on a 20" rifle. I dont recall what the mid length port pressure is. I tend to like the looks of the mid lengths more in general, and like the longer handguard. Some people say they think the mids shoot a little softer than carbine length. I'm weird though, I like the 20" guns, and they do shoot softer than the shorter guns.

Actually, I think the carbine length was even before 14.5" barrels. Like the 1960's and the first Colt shorties with about 11 1/2" barrels.

 

[Just Us]

Only possible benefit I can think of is that it would cycle pretty much any ammo you put in it.

Otherwise, no.

Middy is the way to go on a 16".

 

[MtnCreek]

Do manufacturers set port dia based on barrel length?

 

[Sam1911]

Do manufacturers set port dia based on barrel length?

Hard to speak for all manufacturers, but a well built gun will have a properly sized port for the barrel length (primarily here the barrel length in out in front of the gas port) and gas system length.

 

[briansmithwins]

I really, really prefer midlength gas for 16" barrels. Less port pressure without as much dwell as carbine gas.

Plus I think the proportions are better than carbine length gas on a 16" barrel.

BSW

 

[Bartholomew Roberts]

Actually, I prefer midlength for 14.5" and 16". The closer the gas port is to the chamber, the faster it erodes out it seems. However, you need proper dwell time as well, so it is a bit of give and take.

Still, I think the average shooter would never notice any difference between the two assuming proper port size, buffers, springs, etc. And I'd add that a midlength badly set up on port size, buffers, etc. can run worse than a properly set-up carbine.

 

[Fishbed77]

Been reading articles covering AR-15 gas system lengths. Seems that the carbine length system is intended for 14.5" barrels and tends to be rough on the gun with 16" variety.

Are there any benefits to having a carbine system with a 16" barrel and are the claims above exaggerated?

Yes. The claims are exaggerated.

The science shows less port pressure with less dwell time, but we are talking very small differences in practical terms. I own a midlength 16" barrel BCM rifle, and one can perceive slightly less recoil.

But Colt 6920s, 6720s, and others have run just fine for many years with carbine gas systems paired with 16" barrels.

 

[G11354]

Appreciate the feedback everyone!

I couldn't help but be curious as to whether or not it was worth my time to convert my AR over to a mid length barrel. For the time being I'll stick with my carbine barrel and maybe upgrade to a mid length later on down the road.

 

[Malamute]

I think thats a good choice.

Changing an existing, functioning gun? I doubt its worth it. As a choice when rebarreling or building a gun up from parts, or getting a different upper, sure, worth considering.

I believe there is a spec on bolt life for different types (and its lower on carbines than 20" rifles), but most people just run them til they break, if they ever get that many rounds through them at all. Parts are a minor consideration when talking enough rounds to seriously wear a barrel out or start breaking quality parts.

 

[Jeff White]

I have had zero problems with carbine length gas systems on barrels from 10" to 16" in both semi and auto applications. I converted 5 M16A1s to 16" carbines for the sheriff and 2 of them required "H" buffers to function well on auto. Semi auto fire was fine.

The 2 16 inch carbines I own now, Colt 6920 and an identical carbine I built from Colt parts on a Bushmaster lower function fine. Both are high round count weapons, the one I built is used as a loaner when I teach patrol rifle and police carbine courses.

I've never seen the advantage of a "mid-length" gas system on a carbine length weapon. The carbine length gas system has been in use since about 1966. It's been used on barrels from 10" to 16" with the original sound "moderator", standard flash hider, and suppressors.

I'm not going to say there is no purpose for a mid-length system, I just don't see any advantage over a properly set up carbine length system.

 

[Tirod]

While carbine gas has been around for awhile, I don't know of an issue rifle with 16" barrel that uses it.

The M4gery market that became widespread ten years ago used issue M4 handguards and the GI flashhider location on an NFA legal 16" barrel. There were a lot of those made, and the industry came up with midlength as an answer to some CS issues they were having. It meant inventing a new gas tube and handguards to sell it.

While a single user may not see much point to changing out one barrel, when you make thousands the edges of the operating envelope become more clear. Gas length on barrels is optimally determined from it's best timing about 5 inches back from the muzzle. That's taps gas at whatever pressure exists but not so early it creates issues with brass sticking to the chamber wall or causing high cyclic speeds and bolt override. In some cases as documented on video bolt carriers will slam up against the barrel extension and rebound - which is really bad if the firing pin made contact at the point it was locked.

Crane speaks on that when timing the Mk 18 with 10.5" barrel - use up to an H3 as necessary. It also goes to a lot of us building carbines using rifle buffers on 16" barrels. It does slow the action down and incrementally smooths it out, plus seems to reduce cyclic issues.

Point being, the industry went to midlength wholesale on a lot of the subsequent rifles and that changed the M4gery market as quite a few left it behind.

As for the OP - on just one carbine, the expense isn't justified. Shoot it out then change it. But for an industry making thousands of carbines annually, there must have been some justification to increase their expenses to create a new gas length and then offer it. While saying "There's no point as far as I can see." is certainly justifiable on a few guns in use, in the aggregate others definitely thought differently and carbine gas on 16" isn't as trusted as some make out.

 

[HammsBeer]

Carbine gas on 16" barrels compound the gas problem in two ways. First the port pressure is much higher, and the bullet dwell time is too long, so the BCG is getting higher pressures for a longer duration than it needs. Will carbine gas work on a 16", sure tons of AR's are setup like that, but I'd never purposely get one that way.

Carbine gas is great if you have a 14.5" barrel with a permanent flash hider.

Middy gas is perfect on a 16" barrel.

Rifle length gas for 18"-20" barrels.

 

[Jackal]

I've tried my Odinworks adjustable gas block on a few barrels and even on a 14.5" VooDoo midlength, I had to nearly shut the gas off until it wouldnt cycle Tula... and thats using a T2 buffer...

Its my suspicion that MOST barrels run with far more gas than needed. I never noticed the difference, until I actually started to tune them.

 

[Jeff White]

While carbine gas has been around for awhile, I don't know of an issue rifle with 16" barrel that uses it.

The 14.5 inch barrel was specified by the military to facilitate the attachment of the issue bayonet. The selection of that barrel length had nothing to do with the gas system.

Colt introduced the AR15 SP1 Carbine (Model R6001) in September of 1977. Colt has cataloged the following 5.56 mm models with barrel lengths less then 20 inches and they ALL use a carbine length gas system.

R0605A M16A1 Carbine 14.5" barrel
R0605B M16A1 Carbine 14.5" barrel
R0607 SMG Tanker Model 10" barrel
R0607A SMG Tanker Model 10" barrel
R0607B SMG Tanker Model 10" barrel
R0608 Survival Rifle 10" barrel
R0609 XM177E1 Commando (US Army) 11.5" barrel
RO610 GAU-5/A USAF 10' barrel
R0610B SMG 10" barrel
R0619 XM177E1 Export Carbine 11.5" barrel
R0620 XM177 Export Carbine 10" barrel
R0629 XM177E2 SMG US Army 11.5" barrel
R0639 XM177E2 Export Model 11.5" barrel
R0640 M16 SMG 11.5" barrel
R0649 GAU-5/A/A USAF 10" barrel
R0651 M16A1 Carbine 14.5" barrel
R0652 M16 Carbine 14.5" barrel
R0720 M16A2 XM4 Carbine 14.5" barrel
R0721 M16A2 XM4A1 Carbine 14.5" barrel
R0723 M16A2 Carbine 14.5" barrel
R0723S M16A2 Carbine 14.5" barrel
R0723X M16A2 Carbine 14.5" barrel
R0725 M16A2 Carbine - UAE 15.5" barrel
R0725H M16A2 Carbine - Holland 15.5" barrel
R0727 M16A2 Carbine UAE 14.5" HBAR barrel
R0727 M16A2 Carbine UAE 14.5" barrel
R0728 M16A2 Carbine 14.5" M4 HBAR barrel
R0729 M16A2 Carbine 14.5" M4 HBAR barrel
R0733 M16A2 Commando 11.5" A1 barrel
R0734A M16A2 Commando 11.5" A1 barrel
R0734 M16A2 Commando 11.5" A1 barrel
R0735 M16A2 Commando 11.5" A1 barrel
R0735B M16A2 Commando - Brazil 11.5 A1 barrel
R0737 M16A2 Commando 11.5" A1 barrel
R0738 Enhanced Carbine 11.5" A1 barrel
R0770 M16A3 Carbine 14.5" HBAR barrel
R0777 M4 Carbine w/extended feed ramps and heavy buffer 14.5" M4 barrel
R0778 M4 Carbine w/extended feed ramps and heavy buffer 14.5" M4 barrel
R0779 M4 Carbine w/extended feed ramps and heavy buffer 14.5" M4 barrel
R0780 M16A3 Commando 11.5" A1 barrel
R0920 M4 14.5" M4 barrel
R0920AF M4 Carbine USAF 14.5" M4 barrel
R0921 M4A1 Carbine 14.5" M4 barrel
R0921HB M4A1 Carbine 14.5" M4 heavy carbine barrel
R09211 M4A1 Carbine - Israel 14.5" M4 barrel
R0925 M4A1 14.5" M4 barrel
R0927 M16A4 Carbine 14.5" M4 heavy carbine barrel
R0933 M16A4 Commando 11.5" A1 barrel
R0933CQB M4 Commando 11.5" A1 barrel
R0935G M16A4 Commando 11.5" A1 barrel
R0935 M16A4 Commando 11.5" A1 barrel
R0938 M4/M16A4 Commando HBAR 11.5" HBAR
R0977 M4 Carbine 14.5" M4 barrel
R0977CQB M4 Carbine 14.5" M4 barrel
R0978 M4 Carbine 14.5" M4 barrel
R0979 M4 Carbine 14.5" M4 barrel
R0979 M4 NYPD Carbine 14.5" M4 barrel
XM4 Carbine 14.5" A1 barrel
C8 Carbine Canada 14.5" A1 barrel
AR6320 Police Patrol Carbine 16" A1 barrel
AR6520 AR15A2 Government Carbine 16" A1 barrel
AR6520M AR15A2 Government Carbine Mexico 16" A1 barrel
AR6522 AR15A2 Government Carbine 16" A1 barrel
AR6525 AR15A2 Government HBAR Carbine 16.1" HBAR
AR6526 AR15A2 Government HBAR Carbine 16.1" HBAR
AR6530 Law Enforcement Light Weight Carbine 16" A1 barrel
AR6530M Law Enforcement Lightweight Carbine - Mexico 16" A1 barrel
AR6535 Law Enforcement Lightweight Carbine 16" A1 barrel
AR6536 California Highway Patrol Carbine 16" HBAR
AR6721 AR15A3 Tactical Carbine 16" HBAR
AR6731 Competition HBAR II Carbine 16.1" HBAR
LE6920 Law Enforcement Carbine 16.1" M4 barrel
LE6921 Law Enforcement HBAR Carbine 16.1" M4 heavy barrel
LE6921 Law Enforcement Carbine 14.5" M4 barrel
LE6922 Law Enforcement Carbine 16.1" M4 barrel
R6001 AR15 Sp-1 Carbine 16" Standard barrel
R6420 AR15A2 Sporter Carbine 16" A1 barrel
R6425 AR15A2 Sporter Carbine 16"A1 barrel
R6530 AR15A2 Government Carbine 16" A1 barrel
R6530 Sporter Lightweight 16" A1 barrel
R6721 AR15A3 Tactical Carbine 16" HBAR
MT6400C Match Target M4 Carbine 16" M4 barrel
MT6530 Match Target Light Weight 16" A1 barrel
MT6731 Match Target Competition HBAR II 16" HBAR
MT6731C Match Target Competition HBAR II 16" HBAR
MT6731HC Match Target Competition HBAR II 16" HBAR

As you can see Colt has produced millions of weapons in barrel lengths ranging from 10" to 16.1" over the years. Never have they built one with a mid length gas system. They all functioned as designed with a carbine gas system.

Mid length gas systems were developed to correct problems people were having building their M4gerys from out of spec aftermarket parts. They are NOT necessary for a properly functioning weapon if you use the correct parts building your carbine. That means you have to pony up and pay for barrels with the gas port cut to the correct diameter and use the correct buffer spring, buffer and disconnector.

I reject the idea that mid length gas systems are somehow better. Colt, who owned the AR/M16 pattern world never felt the need for them.

 

[boricua9mm]

The first midlength I ever saw was an Armalite, and it was around '99 or 2000. They may have dabbled with it before then, but I wasn't much of an AR guy until that timeframe. Nobody even went near this concept for another several years. Remember, back then, the AR15 family as a whole was not ragingly popular at all, and while the M-Faux was the one to have, it was not the massive/momentous market force it's painted out to be. Seems strange to say it, but the weapons system seems to have developed more since the expiration of the AWB than it did from the decades from the SP1 days leading up to it.

I have more midlengths than carbine gas guns. They all run just fine for me. Softer recoil? I don't notice any difference. The degrees of wear on the bolts seem to be lesser with the middies, but that's all I can detect. At the end of the day, the size of the gas port is going to be more important than any theoretical debate about the advantages of the distance from the port to the muzzle.

One of my rifles has a Daniel Defense 14.5 pinned upper on it, and the things runs Wolf just fine. By many accounts, this gun should not run this ammo at all unless it's "over-gassed." Nowadays, it is commonly held that DD rifles tend to have larger-than-necessary gas ports. Is this a problem? Well, maybe if you are worried about your split times and the brutal, punishing recoil of the mighty 5.56mm.

Despite all of this, the carbine gas system is the proven system at the present time. It has done very well in the short guns, and in the LE and Civy world, it has proven itself in the 16 inchers. While the midlength may be operating at optimal pressures under optimal conditions, I have no doubts that a carbine gas 16" gun will perform better in more severe circumstances (temp. & filth) than an optimized midlength. Do I have the test results to prove it? No, but a sizable data pool of long term middy use isn't quite there to make me change my mind either.

 

[Theohazard]

Changing an existing, functioning gun? I doubt its worth it. As a choice when rebarreling or building a gun up from parts, or getting a different upper, sure, worth considering.

This. In theory, a mid-length gas system is best for a 16" barrel, but in practice the difference will be barely noticable. And if you're having over-gassing issues with a 16" carbine-length system there are much cheaper ways to address the problem than buying a new barrel.

When I build an upper I prefer mid-length barrels unless I'm going shorter than 14.5". But if I already had an upper with a carbine-length barrel I wouldn't bother to change it unless the barrel needed replacing anyway.

 

[Zerodefect]

The carbine can eat weaker plinking rounds and russian ammo. So can a 16" middy, it's the 14.5" middys, and 18" rifles that might be a little picky. Assuming proper port size. But I know a Colt 6920 eats just about anything.

I might run a little more buffer weight in the carbine. Usually I steal a weight from the buffer of my 18" rifle to give to to the carbine 16".

 

[Theohazard]

I guess I should point out that I run a 14.5" mid-length because I shoot mostly suppressed. If I didn't, I would run a carbine-length on anything shorter than 16".

 

[Jeff White]

But for an industry making thousands of carbines annually, there must have been some justification to increase their expenses to create a new gas length and then offer it.

Of course there was justification. They couldn't make their guns run. Colt owns the patents and the technical data packages on the AR15/M16/M4 series weapons. Why do you think that you could not buy a complete AR pattern rifle from another manufacturer until sometime in the late 80s or early 90s? It wasn't because the market wasn't there. It was because Colt successfully sued a couple companies who tried to build and market them. Around 82 or 83 Rock Island Armory (I'm not certain if it's the same company that goes by that name now) was trying to market several versions of AR pattern rifles. I know, because I ordered one from my local dealer. I never received it. Colt sued them and to my knowledge none of the rifles or carbines they advertised were ever released.

At that time a Bushmaster was a bullpup and they sold their AR/M16 parts under the name Quality Parts Company. It wasn't until patents that Colt purchased in the late 50s ran out that anyone started selling complete rifles.

But even though the patents ran out, the new companies didn't have access to all of the technical data that Colt spent millions (and a lot of those millions were tax dollars) developing. So they winged it. Tried reverse engineering, guessing and when the M4gery market started growing by leaps and bounds a lot of the smaller manufacturers that didn't have the engineers on staff, the sophisticated test equipment and 30 years experience building these weapons were having troubles because their carbines wouldn't run.

Too much gas led to bolt bounce on semi, too little gas led to short recoil. I saw it all in classes I took or taught. The only 16 inch guns that you could count on to run reliably were Colt. Yes some of the others would run but as a whole, if you wanted a carbine to be reliable you bought a Colt. Or you built one with Colt parts. It wasn't until about 2005 that other companies started catching up with Colt in the reliability department. Of course those companies were the higher end of the post Colt producers.

The other post Colt manufacturers solved their problems by creating the mid-length gas system.

When someone can come up with a plausible explanation as to why the Colt 16" guns have worked since 1977 and why they never had to go to a "mid length" gas system to get them to run, and why Bravo Company guns and Noveske's guns run, but others require a mid length gas system to run reliably I will listen and maybe change my mind about mid length gas systems. But my personal experience with 41 years professionally using the M16/AR15/M4 system tells me that it's not necessary to have a mid length gas system on a 16" barrel carbine. You just have to have a 16" barreled carbine that's properly built.

 

[Bartholomew Roberts]

I couldn't say for sure who developed the midlength gas system. I'd be surprised if Colt did not experiment with different length gas tubes at various points; but I think Jeff may have it backwards. I think the civilian community started working on recognized problems with the M4 and implemented the midlength gas system before Colt and the Army could come to an agreement on formal M4 fixes. The first manufacturer to bring midlength gas systems to the civilian market was Armalite (the rebirth, not the original).

Lt. Col. Mark Westrom (ret), was the founder of Armalite 2.0. He was commissioned as a U.S. Army Ordnance officer in 1973 and was very active in the match shooting world as well. He mentions that his job - armament maintenance officer - doesn't even exist anymore. He was also at the forefront of bringing the M16 into the National Match competition, which was dominated by the M14 for most of the time he was active duty.

He purchased Armalite in 1994 - just 9 months before the Assault Weapons Ban was enacted - with the goal of building National Match AR15s for competition. Armalite was one of the first companies to offer a midlength gas system - and in fact, that was the standard available on their carbines.

About 6 years prior to the rebirth of Armalite, Colt had begun the program that would lead to the M4 Carbine - though large numbers of them wouldn't hit the military for some time still. It was primarily a Special Operations type weapon as it combined the compactness of the old Colt Commando with the ability to mount the M203. The first big purchases went out to UAE in 1993 IIRC.

During 1993-2000, it was recognized there were some shortcomings in the M4 system. By 2001, these shortcomings had been identified and fixes had been proposed - upgraded extractor springs, different extractor spring buffer, upgraded buffer - pretty much all of the same shortcomings that Armalite addressed in 1994 via a longer gas tube. So you could argue that both are just different approaches to addressing the same problems observed in longer barrelled M16 type carbines.

By 2004-2006, the midlength was gaining a lot of popularity in the civilian world; but on the military side, the M4 was now probably the most well-tested and well-understood rifle ever fielded by the military. The M4 had been the subject of numerous tests to destruction, accuracy tests, reliability tests, etc. At this point the M4 (and carbine length gas system) was a well understood system with a giant knowledge base behind it.

Whether the original Armalite approach had more merit or the Colt approach did in addressing the original issues of the M4 is pretty much a moot question at this point. All of those issues have been addressed in either system - and if you buy some commercial rifle that cut corners, it is relatively easy to correct it. Heck, one of the problems I see more nowadays is the "if a little is good, a lot is better!" approach where people combine all the reliability enhancements from a carbine with a midlength gas system and enhance themselves right into a non-functional state.

Personally, I think Armalite's approach was the superior one. In high round count rifles, the gas port gets sandblasted by unburnt powder - and the closer the port is to the chamber, the worse the effect is. If you section a carbine barrel after it is shot out, you'll see the difference in port erosion. From a practical standpoint, all that means is you may need to adjust to heavier buffers over the course of a carbine's barrel life. So, not exactly a show stopper. And midlengths aren't immune to gas port erosion either; but I think long term they are less likely to have issues related to gas port erosion. For 3-gunner types, the midlength system lends itself better to lighter buffers and less reciprocating mass - which is big for not disturbing shot picture during follow up)

At this point, I don't think there is much difference between well-maintained examples of either system in reliability.

 

[Jeff White]

Bart,
To my knowledge the first 16" carbines were the SP1 carbines that were produced in 1977. 17 years before Westrom bought the Armalite name and started building ARs. The only reliability changes that were made to the SP1 carbines were a longer disconnector and a slightly different trigger. When the first big change to the commercial Colt ARs came out in the late 80s, the addition of the M16A2 type upper receivers with the AR15A2 rifles, Colt quickly followed up with the AR6520 AR15A2 Government Carbine, again with a 16" barrel and the same length gas system Colt developed in 1966.

Almost all of the reliability enhancements that went into the M4 development were to enhance reliability in burst and automatic fire. When I converted the M16A1s to carbines for the sheriffs office several years back, the kits they bought from CMMG included 16" heavy barrels and standard carbine buffers of an unknown manufacturer. They all ran semi auto. Two of them would not function on auto, bolt bounce from too much gas. I fixed it by installing Colt "H" buffers in all five weapons.

As for Colt's development of short barreled AR pattern weapons, it goes back to 1964 when they began development of the CAR-15 and CMG-1 5.56mm military weapons system. This was to consist of seven different weapons, the standard M16, a carbine with a 15" barrel, two different light machine guns, one magazine fed and one belt fed, a 10" barreled sub-machine gun with a collapsible stock and a survival rifle.

The factory prototype CAR-15 carbine was built with a 16" barrel and a shorter gas system with a 4 position selector. The photograph of the original from the Colt pattern room in Ezell's The Black Rifle looks a lot like a mid length gas system judging by how much of the barrel is in front of the front sight base/gas block. The production model that they tried unsuccessfully to sell had a 15" barrel and the M16 3 prong flash suppressor butts up against the front sight base. It appears to have the same length gas system as the 20" barreled M16s. It's interesting that on page 171 there is a photograph of a "lightened" bolt carrier that was developed and tested in the shorter weapons.

In early 1966 the Army ordered 2,050 Colt sub-machine guns with delivery to be completed by November 1966. The development problems centered on the huge fireball from the 10" barrel and the charging handle becoming unlatched while firing. The fireball problem was fixed with a Colt designed sound moderator/flash suppressor that reduced the report to approximately that of the standard M16 and the Army went with an 11.5" barrel. The charging handle latch got a new spring.

Most of these weapons went to MACV-SOG where they saw heavy use in combat. I've not been able to find any reports of reliability problems with them firing semi or full auto. I would say that Colt figured out the carbine gas system by 1966.

Is engineering so much better now, or did the aftermarket carbine manufacturers have to go a different route to get their carbines to function because they didn't have Colt's decades of institutional knowledge? Like I said in my earlier post, Colt figured this out in the early 1960s. Their carbines, no matter what length barrel, 10", 11.5", 14.5" 16.1" all run. And they don't use a midlength gas system on any of them.

 

[Quentin]

Excellent information Jeff and Bart! I enjoyed reading your very knowledgable posts.

I would say Jeff's information applies well to select fire rifles and Bart's more to semiautos available to the average citizen. However, midlength gas does well for a full auto carbine but the military is locked into carbine length gas. It would present compatibility problems using both gas systems in similar carbines. Smaller entities that standardize on select fire midlength gas carbines would do fine.

For the rest of us buying a street legal AR carbine we can go with either gas length. I like the ArmaLite midlength solution to 16" ARs but sure wouldn't go to the expense of replacing a 16" barrel with carbine length gas. Anyone buying a new 16" AR should read and understand both schools of thought regarding gas tube length.

 

[Bartholomew Roberts]

Is engineering so much better now, or did the aftermarket carbine manufacturers have to go a different route to get their carbines to function because they didn't have Colt's decades of institutional knowledge? Like I said in my earlier post, Colt figured this out in the early 1960s. Their carbines, no matter what length barrel, 10", 11.5", 14.5" 16.1" all run. And they don't use a midlength gas system on any of them.

Well, while the 16" SP1 with carbine gas system has been around for a long time, I can't think of anyone who was putting it through heavy firing schedules where problems with it would be evident prior to 1990.

If you look at the original problems reported with the M4 (say 1994-2002), there are several problems (non-gas tube related - such as cooks offs, barrel bursts), that are virtually never seen on the civilian side even now with 5-day carbine courses; but certainly not then. Those problems point to a heavy firing schedule by military users who were previously using the shorter Commando carbines where a midlength system was not even an option (and would have created dwell time issues had it been used).

Until 1990 or so, you don't really see M16 carbine variants with barrels longer than 14.5" in common military use. My earliest recollection is a very early M4 in an issue of "All Hands" magazine featuring Rudy Boesch as the senior enlisted SEAL in the Navy. That would have been prior to the UAE order; but only by a few years.

Once you start seeing the longer barreled carbine variants, you also start seeing reports of problems related to too early extraction - although again, you're only seeing them from units training hard under a heavy firing schedule. Which brings up another issue from the mid-1990s to 2002 or so... during that time, you basically have a broken military maintenance system and unrealistic expectations of firearms service life. I can remember one of the former Platoon Sergeants of Capt. Nate Self's unit involved in the early Afghanistan fighting talking about how they deployed with M4's that had in excess of 80,000 rounds fired through them in training and that the reason they taped cleaning rods to the side to extract stuck cases was because they had identified that issue in training. Likewise, I can remember SEALs complaining that they would start their six-month work up with a new M4; but that it would be near shot-out by the end of the work-up and then they would have to deploy with the same marginal rifle. From more recent conversations, I gather that the military eventually became more realistic in their expectations of service life and the required maintenance; but at the same time a lot of these problems are reported in the M4, you've got some questionable maintenance based on unrealistic expectations higher up the chain.

I'm not sure when Westrom left active duty to go reserves; but if he was an ordnance officer still in the early 1990s (or in that area of work), it is possible he was aware of the problems, even if he might not have been aware of Colt's evolving solutions for those problems. Also, Colt took the position (successfully) in court that the M4 TDP was a completely different TDP from the M16 and that their rights to it did not expire when the M16 rights did. They sued the United States successfully over that point after the U.S. handed out the TDP to several other manufacturers (including Bushmaster) in the early 1990s during the Gulf War. So even if Westrom was aware of the issue and the technical solution to it; he might have been unable to pursue that technical solution due to the legal issues and instead forced to look to other solutions like a midlength gas system.

I'd certainly be interested in hearing Westrom discuss the choice to go to a midlength gas system in the mid-1990s for his civilian ARs; because I'm pretty sure that this - along with the Armalite Tech Note justifying it - are a big part of why the midlength is popular with civilian shooters today.

As somebody who has shot out some carbine barrels, I prefer the midlength system* myself; but even if I deliberately shot out every midlength I had and then subjected a carbine system to the same firing schedule, it would still be more anecdote than data. The one thing the carbine system has going for it, is that due to the massive amount of testing of the M4 carbine - you've got plenty of actual data to review as opposed to a few anecdotes to put together.

*Having said that, if I had a carbine barrel - I would shoot it until it stopped performing for me before I considered changing it out. On the last carbine barrel I had, it stopped performing accuracy wise at 300m long before any gas port issues manifested - though that rifle did use a MGI rate reducing buffer that was heavy enough to choke an unsuppressed midlength entirely - and the carbine had no problems with the MGI buffer, which probably suggests that it was getting more gas than it needed.