The Case For The 20″-Barreled AR-15
BY Herschel Smith
5 years, 10 months ago
First, let’s look at the ballistics. The M16’s 20″ barrel has a 200-f.p.s. advantage over the 14.5″-barreled M4 when shooting M855 ammunition. In my testing, using Federal’s XM855 ammunition, a 20″ barrel recorded about a 150-f.p.s. advantage compared with a 16″ barrel, the common length for civilian carbines. For the carbine, that means about a 5 percent loss in velocity. The muzzle energy difference is about 125 ft.-lbs. or 10 percent.
For reliability and durability data we can look to tests conducted by the U.S. military, which give an edge to the M16 over the M4. The main reason lies in the gas system. The “rifle-length” gas system of a 20″ barrel is 5″ longer than the “carbine-length” gas system used on all 14.5″ and many 16″ M4-style carbines. Due to the drop in pressure over this longer distance, the gas port on a rifle can be larger, which results in a larger volume of lower-pressure gas heading back to the action. The extra length of the gas tube also means the velocity of the gas is slower when it reaches the bolt carrier. This means less force and heat on the working components of a rifle’s action. In contrast, the shorter length of a carbine gas system means the bolt is unlocking sooner, while chamber pressure is higher, which results in more stress on bolt lugs and extractors.
While the contemporary M4-style carbines have evolved into a highly reliable platform, it was a process that was not without its teething problems, a path marked by the necessity of innovations such as mid-length gas systems, extra-power extractor springs, modified feed ramps and H (heavy) buffers. The bottom line is that, for the first three decades of its existence, the M16/AR-15 rifle, and its 5.56×45 mm NATO cartridge, were developed and refined around a 20″ barrel. Anything shorter is a compromise.
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Soon after it adopted the M16, the Army saw the need for a bolt-closing device, as the rifle’s nonreciprocating charging handle can only be used to pull the bolt to the rear. The XM16E1 added a forward-assist device on the right rear of the upper receiver, and the design was standardized in 1967 as the M16A1. Stoner was clear in his belief that the forward assist had no place on the AR-15 design.
“The rationale was if the weapon was dirty enough or has sand or dirt or mud or something in it and doesn’t close, the first immediate reaction should be to open the bolt and try to find out the cause of it, and not beat it shut and then find out you’ve got a disaster on your hands,” Stoner pointed out to Ezell.
I’ve never used my forward-assist in any rifle, and I have to believe that use of it in the early stages of use (e.g., Vietnam) was because of inferior parts, teething pains, and lack of attention to the machine. I’ve also never had a single malfunction, whether FTF, FTE, or anything else, and can say that having shot thousands upon thousands of rounds downrange. We’ve done much better now with better parts and better builds.
Every gun choice is a compromise, and I happen to think that 18″ is a good compromise, while having a short-barreled carbine is good for CQB.
On February 18, 2019 at 10:38 pm, MTHead said:
I always thought the forward assist was for loading the gun quietly. as lowering the bolt slowly on a full magazine will not fully lock the bolt.(in most cases). you can “walk” the bolt forward to full lock.
Stoners right, beating crap down into the chamber is not going to work. if the gun won’t cycle on its own, find the problem and fix it.
Otherwise its a “forward jam” button.
On February 19, 2019 at 1:19 am, Georgiaboy61 said:
The debate about barrel length naturally circles back around to the type of ammunition being fired and the task for which the firearm is being used.
Eugene Stoner’s original 20-inch barreled AR15 – which ultimately came to be the M-16 – was premised upon a full-length rifle barrel. At the time in the mid-1950s, twenty inches was considered fairly short for a combat long arm; the M-1 Garand had a 24-inch barrel and the M-14 a 22-inch tube.
Stoner and his design team had been tasked with developing a light combat rifle – an assault rifle – which would be lighter and more compact than the designs which came before it, and which would fire a cartridge which – although equivalent in lethality at the most-common ranges for infantry combat (0-300 yards) to .30-caliber cartridges such as the 30-06 and 7.62 NATO before it – would be considerably smaller and lighter than those cartridges.
That’s a tall order from an engineering and design standpoint. Stoner and company devised a work-around which was ingenious, but did involve a few compromises. In order to make the smaller and much lighter 55-grain .224-caliber bullet as lethal as its much larger .30-caliber 150-grain counterpart, a ballistician or ammunition designer would normally use a hollow or expanding point to enhance the lethality of the smaller projectile, or perhaps make it frangible like a varmint bullet would be.
However, those kinds of modifications are illegal – under the Hague and Geneva Conventions – for ammunition used by nation-state militaries, which mandate that only jacketed (full-metal jacket) projectiles may be used, i.e. a lead core surrounded by a gilded metal sheath.
In order to obtain an effect like a HP or expanding bullet, Stoner and team instead designed the 55-grain projectile with a crimping groove or cannelure, which – when the bullet was propelled at sufficiently high muzzle velocity – would fracture into a high-velocity blast cone of fragments upon encountering a target. This design feature allowed the U.S. military to remain within the confines of the Geneva and Hague treaties, while still having the desired combat effectiveness.
Indeed, the U.S. was not the only nation to experiment around with small-arms ammunition with an eye toward improving its lethality during wartime, while still remaining within the confines of the agreed-upon treaties. Britain had already designed their bullets for the legendary .303 Lee-Enfield rifle to have a cavity in the tip containing aluminum, a metal much lighter and less-dense than lead. Upon encountering a target and ceasing to spin-stabilize, the heavier end would swing around to replace the lighter tip, thus widening the wound channel and perhaps fragmenting as well. Insidious stuff.
The bullet shattering effect of the AR15 M193 55-grain bullet was seen reliably only at high MVs, of perhaps 2700-2800 fps or more – which necessitated a rifle-length barrel to assure as much usable muzzle velocity as possible, and maximum optimal combat effectiveness as well. Hence the 20-inch barrel of the original design.
The M855 62-grain “green tip” round which replaced M193 as the NATO standard in 1980, was – like its predecessor – dependent upon high MV for optimal combat effectiveness. It contained a steel cup or “penetrator” embedded in the lead of the mid-forward part of the projectile, a design feature intended to enhance penetration of light sheet metal and steel, such as that found in a standard Warsaw Pact helmet.
The specified MV of the older M193 round was 3260 fps from a 20-inch barrel, whereas the newer and slightly heavier 62-grain projectile clocked in at a slightly slower but still very respectable 3110 fps. Like its predecessor, M855 was highly-dependent upon high MV for its combat performance.
Around the time of the famous “Blackhawk Down” incident in 1993, also called the Battle of the Black Sea or the Battle of Mogadishu, reports began surfacing about M855 not being a reliable fight-stopper in combat. Elite Delta Force and Navy SEAL operators were reporting Somali militiamen taking 3,4, 5 or more center-mass hits while still remaining in the fight. These reports and others prompted a lot of R&D, both within the government proper and in the private sector.
These concerns surfaced again during the Global War on Terror. However, in the ensuing time period, a great deal had been learned by engineers, scientists and technicians – to say nothing of the military users themselves – about the 5.56×45 NATO cartridge and what made it tick.
Moreover, elite door-kickers and commandos of various kinds were increasingly using short-barreled rifles (SBRs), shorty carbines with barrels as short as ten inches – for CQB, house-clearing, and other specialized missions. Often in night-time operations using NVGs and IR vision equipment, visible or infra-red target-designation lasers, and suppressed weapons shooting subsonic ammunition.
The R&D establishment responded with a number of innovations which changed the rules of the game, in particular regarding barrel length and the kinds of muzzle velocities needed to ensure optimal performance in battle. There was also an important legal ruling made.
The legal eagles in the JAG Corps had ruled that so-called “open-tipped match” or “OTM” bullets were legal for use in land warfare as they were not forms of expanding or hollow-point ammunition. This freed the military to issue OTM loads to personnel as they saw fit. Special ops personnel had discovered that heavy, slow-moving bullets worked extremely well in suppressed SBRs and carbines of the type they were using. And these loads were not velocity-dependent for their terminal performance.
The forward part of the bullet fragmented reliably even at low velocities, but the tougher latter half of the slug retained structural integrity sufficient to ensure penetration. This is what ultimately became the SOST round, also called Mk. 318 Mod. 0 – whose innovation was to use a front OTM design with a lead core for the front half of the projectile, but a solid brass back-half, i.e. OTM-RP – “Open-tipped match-Rear Penetrator.”
The 62-grain Mk. 318 Mod0 projectile, a joint development of Federal-ATK and the Naval Special Warfare Center-Crane, Indiana, is barrier blind and performs well against hard or soft targets, at a much higher range of useful MVs in comparison to the old M193 and M855 (SS109) rounds.
Long story short, if you are running M193 or M855, a longer-than-carbine length barrel may be called for – depending on your mission or task. Whereas if you are using some of the newer and more-technologically-advanced cartridge and bullet designs, you may not need that 20-inch tube after all.
Times change and so do people’s tastes. Just read someplace that the young Marines and grunts in the Army these days call 20-inch barreled AR15s/M-16 “muskets” – a term of ridicule for something which is seen as obsolete.
On February 19, 2019 at 6:10 am, sobiloff said:
Another take-away is that the carbine-length gas tube is bad. A 14.5″ barrel using a mid-length gas tube dramatically reduces the pressure, yet still ensures reliable performance. Using Mk 318-style ammo ensures excellent terminal performance at the short (0-300m) distances originally envisioned.
Also agree with MTHead—you can use the forward assist to quietly ensure the firearm is in battery after a chamber check. That wasn’t the original design purpose for it, but the capability sure comes in handy.
On February 19, 2019 at 6:30 am, Nosmo said:
Also figuring in to all this is twist rate – higher velocity (lighter projectile, longer barrel) means it can get by with a slower twist rate, which is why way back when 20″ barrels were 1:12 for 55 grain projectiles. That’s still a workable combination.
Shorter barrels means lower velocities, as does heavier bullets, so twist rate has to increase, especially when shorter barrels and heavier projectiles are used together, hence the 1:7 to 1:9 rates we’re seeing now (1:8 seems to be well accepted as a fairly decent “universal” twist rate for 5.56X45).
The fact remains if one can successfully use a longer barrel there’s no good reason not to and, as stated above RE: gas pressures, some good reasons to use it. Barrel swaps are easy to do with the AR design.
Than again, if one can withstand the weight penalties and additonal recoil associated with it, there’s still a lot to recommend .30 caliber cartridges in the 51 and 63MM lengths…..
On February 19, 2019 at 7:39 am, John said:
I used the A1 back in the early 70’s. 400 rounds and jam,
400 rounds and jam, like clockwork. Yeah, it needed
the forward assist especially after the nightmare in the late 60’s
of jamming on the battlefield with dead troops with their cleaning rods
pushed down the barrel. Yeah, I know it was in part caused by use of powder not
designed for the rifle, but by then many soldiers had lost a lot of faith in the reliability
of the gun and I think the assist was needed to give a psychological boost
as much as a bolt boost.
On February 19, 2019 at 9:48 am, dad29 said:
During Army BCT late ’69, we were told “ALWAYS use the forward assist.”
On February 19, 2019 at 11:21 am, Andy said:
i was trained to use the forward assist on initial load to make sure it was seated.
After that, no.
On February 19, 2019 at 6:37 pm, George 1 said:
I have talked to people with pretty good knowledge of the M-16. Many say regarding the testing about 15 years or so ago to try to replace the M4, that if the M-16 had been in the competition it would have probably gone down differently. That is to say, the M-16 is close to as reliable as any of the top contenders. I assume the SCAR and H&K.
On February 19, 2019 at 7:59 pm, Georgiaboy61 said:
@ sobiloff
Re: “Another take-away is that the carbine-length gas tube is bad. A 14.5″ barrel using a mid-length gas tube dramatically reduces the pressure, yet still ensures reliable performance.”
Yes, a very good point. The shorter the barrel gets on an AR-type carbine or SBR, the harder and hotter the whole system has to run in order to function.
Most people who have shot the traditional 20-inch barrel AR15/M-16 versus the M-4 carbine and its variants know that the longer barrel with its rifle-length gas system is softer-recoiling and that the rifle is easier on its substituent components than a carbine, let alone an SBR.
The dwell time for the longer-barreled variants is greater, which translates into more time for the action to dissipate heat and debris from the previous shot (“more time” being highly-relative; we’re speaking of very small fractions of a second here).
The bolt-bolt carrier group is stressed less, too. Since the operating pressures on rifle-length gas systems are so much less than SBRs, their gas rings and other consumable components do not need replacement as often. The same holds true for the buffer assembly and recoil spring.
The ingenious use of mid-length gas systems on carbines has been a boon to reliability, enhanced usability, and lesser-recoil for these weapons.
@ Nosmo
Re:”Than again, if one can withstand the weight penalties and additional recoil associated with it, there’s still a lot to recommend .30 caliber cartridges in the 51 and 63MM lengths…..”
I was fourteen when the last Huey chopper lifted off the roof of the American embassy in Saigon in 1975, so I wasn’t a participant in that particular “adventure” – but when I was a kid in the late 1960s and 1970s, almost all of the grandfathers, fathers, uncles and older brothers were veterans of service in WWII, Korea, Vietnam or the Cold War. A few were even WWI veterans (yeah, I’m that old!).
I was taught to shoot and how to safely handle a firearm by these men, foremost my father (WWII veteran, USN), but by others as well. For Cub Scout outing, the dads and sons took a field trip to Fort Benning, GA. “The Home of the Infantry.” We boys learned to shoot on BB-guns, pellet guns and lever-action .22s and then graduated to more-adult center-fire fare when we were ready.
The men of those generations, maybe with the exception of some of the younger men who’d been to Vietnam, didn’t really care all that much for the Armalite. They preferred the firearms on which they had been trained – M1911 .45 ACP pistols, M-1 Carbines, Springfield M1903 and M1903A3 bolt-action rifles, and M-1 Garands. On the civilian side, they mostly used Winchester M70 or Remington 70 bolt-action rifles for things like deer hunting.
Long story short, because of the influence of those men, I was a late-comer to the whole AR thing. When I came up, a serious rifle was made of wood and steel, not plastics and aluminum. My thinking changed and modified as time went on, but those old influences remain.
There’s just something special about those old war-horses made of wood-and-steel, when machinists were artisans as much as technicians and before CNC machines came along.
I know there are a lot of surviving combat veterans – infantrymen and people like that – walking around who are alive today only because of that trusty Garand or other old-school arm that kept them alive when things got dicey. An uncle on my wife’s side is one of them. In his eighties now, Uncle Charlie flatly says that if not for his Garand, he wouldn’t be alive today – and that he’d have bought in Korea in 1951 instead. Powerful stuff.
On February 19, 2019 at 9:20 pm, Georgiaboy61 said:
Re: “On the civilian side, they mostly used Winchester M70 or Remington 70 bolt-action rifles for things like deer hunting.”
That’s “Remington 700,” folks….sorry for the typo….
On February 20, 2019 at 12:59 pm, billrla said:
Georgiaboy61: From a fellow ’61, everything you wrote sounds familiar. Reminds me of the old WWI vets, tending bar and selling hotdogs down at the local VFW, which was on the grounds of my town’s then-brand-new municipal swimming pool. I spent my childhood playing war against the Krauts.
On February 20, 2019 at 4:57 pm, Georgiaboy61 said:
@ billrla
Different world today, isn’t it? Those days were a long time ago, but I remember them like it was yesterday. Childhood was still pretty cool in those days. No “helicopter parents” or any of that nonsense. We were what they now call “free-range” kids! It was great. I feel fortunate to have caught the tail end of that old America as I grew up. Not everyone as so lucky…
On February 22, 2019 at 3:53 pm, Bram said:
I spent a good chunk of my 20’s carrying an M16A2 and had lots of complaints. I despised that direct impingement system and would have gladly traded it for a piston gun even if it was a little heavier. I experienced jams when it was around sand, particularly the powdery stuff in the Middle East.
I never understood why the M4 had an adjustable stock but not the M16. It was much more comfortable to shoot without body armor than with.
What I never complained about was the length of the rifle or the barrel. Since Marines have to qualify at 500 yards and in the Gulf War we had fights way past that distance, I wanted a full-length gun. (I did try to trade it in for a M-14)