EFFORTPOST
Direct Impingement AR15s, vs. Piston-Converted AR15s.
This is the work of one evening and coffee, and is undergoing edits. I'm sure it can be revised and expanded.
The AR15 is the first and most common of the modern Direct Impingement semi-auto rifles.Let's examine how this system works when firing very briefly, as it's the basis for the post I'm writing.We're mainly concerned with steps #4, #5 & #6, but all of them play a part. The cycle of operations is as follows -
1. Feeding - The face of the bolt pushes a round from the magazine lips onto the barrel extension.
2. Chambering - The round enters the chamber of the barrel under forward pressure from the bolt.
3. Locking - the bolt rotates on the cam in the bolt carrier, which locks the lugs into the barrel extension, and locks the rim of the extractor over the edge of the case.
4. Firing - The trigger is pulled, sending the face of the hammer into the rear of the firing pin. The firing pin strikes the primer, igniting the powder in the case. The gas from the ignited powder expands rapidly, and forces the projectile down the barrel. The escaping gas behind the projectile is tapped at the gas port partway down the barrel, and delivered by gas tube to the bolt carrier group.
5. Unlocking - The gas pressure in the bolt carrier group unlocks the bolt lugs from the receiver extension lugs.
6. Extraction - The bolt travels rearward, and the extractor grips the rim of the case to the bolt face, and pulls it from the chamber.
7. Ejection - The spring-loaded ejector on the face of the bolt pushes the empty cartridge case out of the ejection port to the right.
8. Cocking - The bolt carrier pushes down the hammer and cocks the action again.
This resets the cycle of operations, as the bolt will now either feed another round, or lock back on empty. As I stated before, most of what we're going to discuss is related to #4, #5 and #6, partly because #4 is a more complex step in a semi-auto than in a bolt action, or pump action long gun.
To see this cycle of operation in full cutaway animation, feel free to click this silent, 37-second youtube video.
Best viewed full-screen.
Also, two large, (2-5 MB) high-quality wireframe GIF files of the same function. Too large to embed, IMHO.
* What do we mean when we say that the AR15 family of firearms is Direct Impingement? In a semi-auto rifle, the action is automatically cycled by the force provided by the firing step of the cycle of operations. There are a few methods that are outside of the scope of this discussion, so I'm just going to cover the Direct Impingement design (DI) and compare it to modern piston designs. In the AR15, the action is cycled by the gas produced by the ignition of powder in step #4. This gas, a very small percentage of the total gas produced, is tapped from the gas port, a small hole in the top of the barrel, from 7-13 inches from the chamber. The gas travels through the gas block, and down a stainless steel tube : the gas tube, into the gas key in to top of the bolt carrier.
The hot gas presses forward on the rear of the bolt, and backwards on the bolt carrier itself. The bolt rotates on the cam pin in the bolt carrier, and is under forward pressure against the locking lugs, from the same hot gas pushing back the bolt carrier. At the time the gas is first entering the gas port, the barrel is also full of hot, VERY high pressure gas. This unlocking process takes very little time, but it is a delay of a few hundredths of a second, which allows enough time for the gas in the barrel to expel the projectile from the muzzle, and for the gas behind it to escape, dropping pressures enough for the bolt to safely unlock from the locking lugs in the barrel. The bolt unlocks, the extractor pulls out the rim of the empty cartridge case, and pulls it back until it ejects.
Bolt locked into barrel extension:
* Be aware that if the bolt were to unlock too early, while the projectile was still in the barrel, the gun would catastrophically fail - it would blow up under the pressure, which is several times higher in the barrel than after the projectile exits. In simple straight blowback guns, such as 9mm carbines, the gas pressure in the chamber pushes back a heavy massive bolt, which along with spring pressure, delays the cycling of the action. This would not work in rifles firing 5.56mm pressure cartridges or larger, as the chamber pressure would require the bolt to weight several pounds and/or be opposed by a 60lb+ action spring. Clearly, the locking delay is required for pressures to drop.
* Now, the bolt itself IS the piston in the Direct Impingement system, which is why it has gas rings around it.
Note the gas rings by the tail of the bolt:
Direct impingement does not mean, as so many have surmised, "without a piston" but rather, that the design is that the gas operates (impinges) on the piston inside the bolt carrier, rather than outside of it as is the case with piston rifles. On the cutaway bolt below, you can see the the gas is directed directly on the bolt(piston) just behind the gas rings.
For the purposes of simplicity in this discussion, though, we're going to refer to the standard, regular Direct Impingement piston as "Direct Impingement", and other conversions that use secondary pistons in place of the gas tube as "Piston rifles"
What makes this system work well, when it does? Correctly setting up the appropriate amount of gas pressure to cycle the rifle. Let's talk about how this is accomplished in various sizes of rifles.
The amount of pressure left in the barrel at the time the bolt unlocks is dependent on a few factors in the rifle's design, leaving out variations in ammunition. The gas pressure can be adjusted on the AR15 in a few ways :
1. by making the gas port larger or smaller,
2. by lengthening or shortening the gas tube,
3. to a lesser degree, by lengthening or shortening the barrel past the gas port.
Ideally, an AR15 would have just enough gas to cycle the rifle and no more. As this is a military design, and variation in ammo, cleanliness, lubrication and wear are must be accounted for, the properly set up AR15/M16 platform has a little more gas than it needs to cycle under perfect conditions, but not hugely more. This can be difficult to calibrate to account for all situations, and the shorter the rifles become, the less margin of error there is built into the cycling of the action. The rule of thumb is that the lower the pressure at the gas port, the more slowly the rifle will cycle. Larger gas ports = more pressure. Shorter gas tubes, nearer the chamber = more pressure. Lots of barrel in front of the gas port = slightly more pressure. This last is known as dwell time, and its effect on pressure has been overstated compared to gas port and barrel pressure variables.
An AR15 can have a little too much gas enter in the gas tube upon firing. Such is the case with rifles with 16" barrels with carbine gas systems, or overly large gas ports, very common in the commercial market. The gas is tapped closer to the chamber at higher pressure, and travels the shorter gas tube faster, causing the operation of the bolt and bolt carrier to be more rapid. In short, the gun cycles more violently, and is considered to be over-gassed. An over-gassed gun is not in danger of exploding, because it still is opening after the projectile has left. However, because the pressure of the gas remaining in the rifle is high, the unlocking happens quickly and under high pressure. The bolt unlocks much faster, and will "grind" more along the locking lugs in the barrel as it moves back. This leads to high bolt velocities and more wear on parts.
* This is an excellent time to interject that the M16, as originally designed, had a 20" barrel, and a long rifle length gas tube ~13" down the barrel. The pressure measured at the gas port on this original design is roughly 13,000 psi, compared to the later M4 with 26,000 psi. The 14.5" M4, though shorter and handier than a 20" rifle, has wear and replacement issues associated with overgassing due to the shortened design, and Colt has issued many incremental improvements as Band-aids to the shorter, higher-pressure platform. They work, but you'd be hard pressed to make the M4 work as long (without maintenance) as an M16A2, all other things being equal. Shorter guns need more care, more lube, and more frequent parts replacement. They unlock more quickly, requiring strong, fresh extractor springs, and cycle more rapidly, requiring strong magazine springs to feed the next round forcefully upward, before the bolt carrier outruns the next round it should feed. With all shorter AR15, the margin of error inside the timing of the cycle of operations is reduced. parts take more abuse, and wear out faster.
"But wait," you say, "my rifle can have any number of heavier springs and buffers installed in the receiver extension, which will slow the bolt carrier travel, and reduce the admittedly already low recoil. Why should I care if the gun unlocks in next to no time at all, as long as it does so after the projectile leaves the chamber?"
Ripped Case Rim:
The answer is extraction. Brass cartridge cases, upon firing, expand and then slightly contract as they rebound and dump heat from the ignition into the chamber walls. The higher the chamber pressures, the less the brass contracts afterward. Also, the hotter the weapon is, the less the brass is able to transfer heat and shrink away from the chamber walls. Conversely, the lower the chamber pressure is, the more easily the brass will come free from the chamber, allowing easy extraction. If your bolt moves too fast, especially in a short, high-gas carbine running warm, then your extractor is more likely to rip the rim off of the cartridge case. This is the extractor claw and the spring behind it, outrunning the ability of your chamber to allow extraction. The buffer and spring you chose will slow the bolt carrier group down, mostly after unlocking has occurred, but it doesn't affect it enough before unlocking to make up for too-high chamber pressures from the front.
"This is all just a long, semi-obvious method of explaining why Colt SP1s are reliable, soft-cycling, and don't require new springs every 5 minutes, as a way of explaining why shorter guns or guns with oversize gas ports cycle faster, more violently, and have a more demanding parts replacement schedule. What does any of this have to do with piston guns?
Good question. The main reason I dragged you through that explanation, is that what makes a good, smooth-cycling, reliable AR15 really relates to what makes piston conversion AR15s less desirable. In fact, I'm going to use a piston manufacturer's animated GIFs to explain what weird and wrong about their system.
In a piston-operated AR15, the process is a little different. The gas is tapped at the gas block, and fills a small piston chamber, which drives a metal tappet rod about the size of a gas tube back into the upper receiver, which strikes a contact point on the modified bolt carrier where the gas key would be. This force drives the bolt carrier group back, unlocking the action, and the cycle of operations continues as normal from there.
Let's look at some rifle bolts designed around piston operation.
AK family:
FN-FAL:
FN-FNC:
M14:
SIG 551:
vz.58:
Look at all of these bolts. They are all different, yet they share a common design theme - big, chunky, rounded bolt locking surfaces. They are designed around being struck or driven by <b>off-center</b> force, by a long or short stroke piston.
This is an AR15 bolt.
The lugs are cleanly squared off, and and are not intended to unlock with any sort of axial, off-center force. The AR15 bolt was designed around the straight-back operation the direct gas impingement action allows for. In short, this is a bolt that was designed to unlock under straight-back gas pressure, and travel straight back through the barrel extension lugs.
* Let me point out some of the deficiencies in the AR15 bolt design that really come into play here. When pushed back under DI gas pressure, it does its job very well. That said, as discussed previously, the shorter your AR15, faster everything cycles, and more things wear out. Where do AR15 bolts take the most wear? The two lugs on either side of the extractor.
These two lugs are undercut on one side, and as there is no lug in the center of the extractor to share the stress of locking and unlocking, they take more abuse than any of the other lugs on the bolt. The faster and harder your bolt is operated, the more pressure the bolt is under as it unlocks, the more likely you are to break one of these two lug somewhere down the line.
Broken lug:
The second place the bolt takes abuse is at the cam pin hole. This is the weakest, slimmest part of the bolt body, and just as before, the faster and harder is it operated, the more likely it is to break.
[IMG]http://i.imgur.com/tR67GFF.jpg[/IMG]
AR15s break parts under normal operations, and most of the show-stoppers are bolt breakages. Regular, old everyday AR15s break bolts. It happens.The softer-gassed guns, such as rifle and midlength gas guns break less bolts than the shorter, overgassed guns with carbine gas or large gas ports. Knights Armament has looked into this design for decades, and developed the proprietary SR15 E3 Bolt, with smooth rounded lugs, no undercut, and a 20,000 round life expectancy. It's no mistake that they ditched the square angles of the original bolt lugs for the rounded ones shown below, and that it uses a longer-than-usual intermediate gas tube for low gas pressure inside the bolt carrier group.
SR15 E3 Bolt w/ rounded lugs
Everything that AR15s do to break themselves, piston conversions make them do faster and harder, with no real advantages. Piston AR15 systems, by their design, do not allow the bolt to unlock or extract as the bolt, bolt carrier, or locking lugs they have borrowed were designed to. The action of the piston rod at the top of the bolt carrier, pushes the front of the carrier up, and the back of the carrier down. Some companies claim to have solved "carrier tilt", with enlarged bosses at the rear of the bolt carrier or angled impact keys where the rod strikes.
Solutions to problems created by other solutions:
This is unfortunately, irrelevant to the real issue of carrier tilt, which is bolt tilt. The AR15 bolt was never meant to be dragged out of the locking lug area with the tail of the bolt forced downwards by the bolt carrier, dragging the top front and bottom rear portions of the bolt lugs along the barrel extension lugs. This causes uneven and unnecessary wear on the lugs, and contributes to premature parts breakage. Also, tilting carriers do cause wear to receiver extensions, which is worth mentioning, even if it is nowhere near as large of a problem as bolts tilting up under pressure, while rotating.
Second, the bolt is designed to have forward gas pressure pressing against the gas rings on the bolt, as the bolt carrier pulls back an unlocks. This provides an equalizing push on the rear of the bolt, and prevents grinding of the rear of the bolt lugs as the carrier moves back. On a piston rifle, there is no gas inside the bolt carrier, and thus no equalizing forward push. This leads to more off center, un-cushioned grinding of the bolt lugs, and premature parts breakage.
Third, the speed at which piston guns unlock is much faster than the speed at which DI guns unlock. This is why they mostly use heavier buffers, to slow the bolt carrier travel. The difference is measured in fractions of a second, but it is significant when dealing with gas pressure. DI guns are designed to unlock as gas fills the bolt carrier group, and provide time for the chamber pressure to drop. The gas piston guns unlock at the speed the rod strikes them, nearly instantaneously, with no cushion of gas pressing the bolt forward and provide the same sort of overzealous extraction that often rips brass, and violent cycling that we expect from a $670 overgassed DPMS. This too, is bad for all moving parts of the gun.
This bolt is not designed to be yanked out fast and at an angle. We work within the limitations of a 50 year old design, do not make it worse.
Fourth, most of the claims made by piston manufacturers are erroneous. Piston salesmen love to tout the "clean, cool design", but notice in the gif above, that the brass never leaves the chamber? Of the two holes that could kick hot carbon into the upper receiver - the chamber and the gas tube - which one do you think is hotter, higher pressure, larger, and nearer to the carbon we're desperately trying to avoid? Yes, it's the chamber. Eliminating the gas tube does not significantly affect cooling or cleanliness, except there will be less carbon on the tail of the bolt. The ONLY major advantage that can be attributed to piston systems, that is not true of DI guns, is that the lubricant remains longer inside piston bolt carrier groups, as it is not being eliminated gradually by the hot gasses flowing by. This does not outweigh the other advantages, and is rectified by occasionally adding more oil to the bolt carrier group.
Are piston guns better suppressed? No, not really. Suppressed rifles have lots of extra pressure in the gas system, because of the gas-slowing effects of the suppressor. The largest hole in the upper, with the most gas, is still the chamber, not the gas tube. You can plug/remove the gas tube on a suppressed rifle, and you will still get hot carbon, gas and filth in the upper receiver, via it's main source, the barrel. Using a charging handle with a shelf or tool-dip seal to redirect/block the gas, and an adjustable gas blocks to reduce the over-pressure, will do more for this than pistons 7 days out of 7. Consider as well, that, unless you have tweaked the gas system just right, your suppressed rifle is overgassed. Overgassing stresses the bolt components. Don't add axial force and speed-unlocking to an already overgassed gun.
Are piston guns better in full-auto? Potentially, yes - in guns shorter than 10.5" where there isn't enough gas to run the gun reliably with a gas tube. Piston guns can run in full-auto, but as always, they are still overstressing their borrowed parts to no benefit, and the gun is not substantially cleaner or cooler than a DI gun. Full-auto is ROUGH on M16s compared to semi-auto, as everything gets hot, stays hot, and operates without the metal cooling, for as long as the gun is firing. Piston operation exacerbates these problems, to no benefit. Otherwise, as Kyle DeFoor pointed out, the Mk18 did it all.
There's no real practical benefit to an M16 shorter than 10.5", as the ballistics on anything shorter are crap, so the entire piston justification can be passed over here as well.
Lastly, these systems are proprietary, follow no standard design, prevent the use of common accessories, are adopted by military forces roughly nowhere, are heavier, add complexity, and additional parts to break. They are the best way to replace a $15 gas tube that works great, with a $400 secondary piston attached to a battering ram, that work ok.
I'm aware that many of our members have owned piston rifles. There are several out there that work - business would be hard to maintain if everyone's stuff broke the first week. I have personally owned and fired 2500 rounds though an LWRC M6A2, and it never gave me any problems. It also wasn't worth having something that was only sort of an AR15, and having to order spare gas system parts from the only place on earth that had them. I would not buy a piston AR15 rifle at a discount, and certainly will never again pay more.
The US military is having overall good luck, apparently, with the current production H&K 416 series, which is a G36 gas piston system force-fucked onto a modified AR15 upper/ lower, using another square-lug AR15-type bolt. It’s more proprietary than a simple conversion kit, and yet, not quite a clean break from the deficiencies of the platform that make for a good purpose-built piston rifle in other designs. More power to them, H&K (and LWRC and Colt, just that I know of) employ retired US Military Generals from the various branches to ensure that their stuff keeps getting purchased. If you had to buy a piston gun, and can get a factory HK416 upper ($3600 on the secondary market and climbing!) it would be a better choice than the others. H&K's 416 is a better piston "AR15" largely due to the fact that it's not really an AR15 anymore, but a complex hybrid with a lot of new engineering. My final word is, if I had $3600 to spend on on AR15, it would not be on a HK416 upper.
If you want a piston gun, get a purpose-designed piston gun, there are many cool new designs - Tavor, SCAR, ARX-160 - and all the older standby piston guns I listed before. There are many excellent choices, many of which are military ruggedized in a way your Adams Arms kit will NEVER be. Don't shoehorn bad, unnecessary, second-piston design into your reliable AR15.
"Wait, why the hell are these things so popular, then? Every third gun I see is a piston conversion of an AR15!"
AR15s are everywhere. They're the nation's most popular rifle with over 200 manufacturers. Nearly every manufacturer of firearms makes an AR15 type, and many do nothing else. In such a crowded market, it's very difficult to stand out. There's bigger names, bigger marketing budgets, and other people were here first. You want to sell a lot of AR15s, but you're new to the game and don't make your parts in-house anyway. How do you establish yourself as cooler than the other AR15 manufacturers?
Well, you could change something - as long as you don't change anything else. The tremendous ubiquity of the AR15 platform, and the enormous aftermarket that has sprung up around it, is a blessing for people selling the "mil-spec" status quo, and a curse to innovators. Why? Because no one wants major components of the rifle to not work in other AR15s. You might have the best idea in the world for a new AR15 trigger, but if it doesn't drop in where the pins are on the standard lower, forget it. You're dead in the water. You might have a great new bolt design that lasts longer, but unless it drops into other AR15s without modification to the barrel extension (or you're Knights Armament), no one will buy it. The problem is, no one wants you to entirely re-invent this rifle, or most of the parts won't work anymore and it might as well be a Robinson XCR.
So what DOES our intrepid breakout manufacturer of AR15s do? He puts a second piston where the gas tube is. It retrofits fairly easily, and he gets to throw marketing jargon about the benefits, compared to "standard DI guns" around for years. Sure, the stuff is proprietary, and yeah, the benefits are untested, but it got his rifle on the cover of Guns & Ammo, and that make him the new POF/LWRC/ADDAX/ARES/PWS/ADCOR/#Pistongun.
Simply put, the secondary-piston system was designed to sell rifles to police departments justifying buying new rifles for the yearly budget, and to people with more money than sense, who felt sure they were buying improvements, because surely nothing that cost more, could be worth less than what it replaced. The AR15/M16 still has people repeating Vietnam-era hearsay anecdotes about how it "craps where it eats" and "if it had a piston like an AK, it'd be good!" This is hilarious, if only because it is our longest-serving and most commercially popular US service rifle.
It takes a special sort of ignorance to look at the 40+ year international service life of the platform and say, "Yeah, direct impingement guns must really suck hard, let's fiddle with the unique operating system!" No. Replace the rifle with the SCAR or something similarly well tested, or make constant material improvements as they have. The people who understand how this rifle works, do not shoehorn secondary pistons on to the top of it.
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External Resources:
Gemtech - EFFECTS OF BARREL LENGTH ON BORE PRESSURE, PROJECTILE VELOCITY and SOUND MEASUREMENT
http://www.dtic.mil/ndia/2010armament/WednesdayCumberlandPhilipDater.pdf
Armalite - TECHNICAL NOTE 48: THE EFFECTS OF BARREL DESIGN AND HEAT ON RELIABILITY
http://www.armalite.com/images/Tech%20Notes%5CTech%20Note%2048,%20Barrel%20Design,%20Heat,%20and%20Reliability,%20030824%E2%80%A6.pdf
AR15Barrels.com Tech Notes
http://www.ar15barrels.com/prod/operation.shtml
Independent researcher with way too much time on his hands
http://www.ewp.rpi.edu/hartford/~ernesto/SPR/LeBlanc-FinalReport.pdf
Relevant credentials, FWIW - AR15 builder since 1998, Paid AR15 builder/troubleshooter 2002-2013, Academi Certified AR15 Armorer.
