DI myths

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http://www.defensereview.com/the-big-m4 ... nreliable/

[tmix]

Interesting read.

This confirms my suspicions that DI guns are extremely reliable. I've never had issues with mine.(other than magazine problems)
I can't tell you how many thousands of rounds I've put through Mongo's LMG without any problems. There is far too much negative lore surrounding the M16 family of weapons. Hell, it's been in service longer than any other long arm in the U.S. inventory. (save the M14)

[Mongo]

Says many of the things I have been posting for a while.

My LMG has never experienced a failure that I can recall and that includes suppressed F/A fire which turned the internals into goo city.

[ick]

Never any problems with my DI system. The SCAR sure is a whole lot easier to clean, but no problems with the DI.

[silencertalk]

In my gun history I have had plenty of AR problems, but I don't remember what they were.

I think part of it is using good ammo, NATO pressure.

Also, like he says - it is all about magazines. Use Magpul Gen-3 or USGI Tan followers.

Mag springs - I would pick USGI 17-7 but I would like to know what the new (2010) USGI spring is.

I am not going to use a random heavy buffer like he suggests, but I do use H2 buffers in carbines.

As for a heavier spring.... I am also not sure that is a good idea. The spring rate was selected after careful testing.

As for an o-ring, that is a good idea in carbines. I am not sure you need a special spring with it though.

As for 4 vs 5 coil, I am never sure people know how to count coils so I don't know what springs he is talking about.

[rob_s]

Something else to keep in mind is that he's using a standard M4 barrel with carbine-length gas system. This is going to put more pressure into the system than a mid-length so if you're trying to replicate his results you need to keep that in mind.

Robert, Dean Caputo says that AR malfunctions are caused by MEAL
Magazines (bad feedlips mostly)
Extractor (worn, wrong material, wrong spring, wrong insert)
Ammunition (low-pressure ammo with too-heavy moving parts or too-small gas port)
Lube (not enough)

My experience matches the above (with my comments added in blue)

Something else I've noticed in ARs over the years, that they have in common with 1911s actually, is that people will buy the cheapest one they can find, with no idea how to run it or maintain it, feed it complete crap ammo from worn-out garbage "pre-ban" magazines, and when they have problems they decree that the whole design is flawed.

[silencertalk]

What is wrong with pre-ban mags?

You mean non USGI steel ones?

Or you mean back/green followers?

What is the max/min on the feed lip separation? I want to check all my mags.

[rob_s]

What I mean is that, especially during the ban, people tend to latch onto magazines and fail to realized that they are a disposable item. If I get repeated malfunctions from any one magazine they go into a box to sell to idiots after the next ban.

Nothing is inherrently wrong with preban magazines, but the well-worn, beat to s--t, pre-ban magazines should be crushed, more often than not.

[-k-]

H buffer for mixed .223 and 5.56 ammo. H2 buffer for 5.56 ammo.

O-ring or extra power extractor spring. Both can "sometimes" keep the extractor from being able to clip over the cartridge rim, maybe the extra power action spring helps.

Beat up mags seem to be the most common issue next to wrong oil/no oil/too little oil.

[silencertalk]

That makes sense on the buffer. The only .223 ammo I have bought was stuff like Black Hills or CMP match ammo. All of my plinking and zombie' ammo is 5.56mm. I tend to use H2 buffers.

The guy said that he shot XXX thousand rounds with no oil. But then he said oil is the key to reliability. I am confused how he ran an AR while dry.

Also, the word on the street is oil is much more of the equation than cleaning the gun. So you have have a gun with 5000 rounds on it and no cleaning, as long as you add oil.

With an o-ring do you keep the black/blue insert in place also?

By the way, the fewer coils a spring has, the more power. So a 4 coil will be more 'powerful' than a 5-coil if the wire diameter is the same. A 4-coil will have room for thicker wire.

How come people speak of 5-coil springs as 'extra power?' More coils is lower stress though.

[-k-]

Keep the extractor buffer in with an O-ring, black or blue doesn't matter.

Extractor springs need to be replaced but the military has no system to track rounds fired or a replacement schedule.

[Mongo]

The so called DI guns are piston guns just like all most all other automatic rifles (few exceptions, HK G3 series, FAMAS, etc), it just has the piston located in the rear of the gun and not up front. This obviously lightens the gun due to the lack of a op rod and it further lightens it since the piston, carrier and bolt are all combined not requiring separate parts. This made the operating mass of the gun very light and the buffer had to be added to control action speed and bolt bounce. The genius of the system is that the piston actually acts against the base of the cartridge during the initial motion of the group. This has a slight sizing effect on the brass to aid in extraction but the main contribution is the linear recoil impulse imparted by the system. That's why a DI M16 is so controllable compared to other guns in its class and given its weight to most of them it is extremely controllable. That's why when the picnic watermelon shoots so swayed the Air Force. There was no gun like it at the time where these days we assume it to be common. Obviously the procurement process and the adoption phases were pooch screwed due to politics and hence the rep of the M16 has suffered to this day.

Does the M16 "s--t where it eats"? - not really, the combustion gasses are vented out the bolt carrier and the magazine, bolt face and feed ramps experience no more carbon build up than your average piston gun. The bolt does get carbon internally where it acts a piston but no more so that the piston face on my Stoner 63A. The big issue is really that the gun will get hot and cook the oil out of the upper receiver making it dry. Proper lubrication and more frequent intervals (than "piston versions") is required and the big difference.

It is hard to directly compare the piston systems to the DI system even though both are piston systems. The piston systems I have shot all have much more energy in them. This can mostly be attributed to excessively large gas ports or incorrectly sized pistons. These guns are not smooth as a DI system and are not pleasant to shoot in many cases. The piston systems usually increase the recoil mass making the gun even rougher under recoil. If you took a DI gun and added the mass to the bolt carrier and buffer and increased the energy delivered by the gas tube you would see very similar results (though a bit smoother due to the in-line recoil) as a piston gun. Its hard to get people to recognize that DI is a piston in a different location and given the same parameters will operate very similarly.


DI guns are more ammo sensitive since gas curves are more critical to them due to the dwell built into the system. Changing to shorter barrel lengths will accentuate this. Given that the average civilian AR15 owner runs random ammo through the guns, probably has haphazardly changed components w/o a clue as to their effects, using unknown quality magazines and most likely incorrectly has assembled their trigger group and now runs KNS pins, its a wonder that they run as well as they do. Notice that when you get to different calibers than 5.56, you do not hear the same complaints about the DI system. In 308, people love it and suddenly no piston system is required and every thing is alright. Another sign of the utter bull s--t propagated about the DI operating system.

Some interesting stories on the reliability question:

http://atwar.blogs.nytimes.com/2010/07/ ... liability/

http://www.defensereview.com/colt-m4-ca ... tographer/

At War, for now, will draw no larger conclusions than this: Whatever the merits of the concerns about the M-4 and the M-16, on the matter of latter-day reliability, the complaints that have boomed on the Web feel out of proportion to what can be documented in the field, and may well be overstated, even hyped.

Sorry morphine ramblings of the day

[Griz]

The real elegance of the AR-15 gas system is that the piston is in-line with the bore.

I like to call the piston systems with an op-rod an "offset piston" to emphasize the key difference.

The offset piston uppers and conversion kits of the last few years solve a very important problem: How to separate gun-nuts from their money. They do that very effectively.

[silencertalk]

http://www.weaponevolution.com/forum/sh ... ng-Upgrade

People seem confused about springs.

This BCM spring is called a '5 coil spring' but it has 4 total coils (2 active coils).

There are also photos of 3.5 and 4.5 coil springs there.

The FEWER the coils, the stronger the spring force.
The MORE coils, the lower the stress - so it may last longer.

So a 5 coil spring is a weaker spring than a 4 coil (as far as extractor tension, but perhaps less likely to break).

That is, assuming the same type of wire - and they don't use the same wire. So there are likely 4.5 coils springs which are stronger than other 4.0 coil springs.

Notice that the Colt spring, which is said to have more force, is a 3.5 total coil spring (1.5 active coils).

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http://www.armytimes.com/news/2007/07/a ... le_070715/

http://www.armytimes.com/news/2007/07/a ... on_070716/

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http://www.armytimes.com/news/2009/11/army_M4_112109w/

The piston gas system performed well in an Army reliability test in November 2007. During the test, the M4 suffered more stoppages than the combined number of jams in the Heckler & Koch XM8; FNH USA’s Special Operations Forces Combat Assault Rifle, or SCAR; and the H&K 416. All three of those weapons use versions of the piston gas system.

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http://www.failzero.com/images/stories/ ... report.pdf

[rob_s]

http://www.armytimes.com/news/2009/11/army_M4_112109w/

The piston gas system performed well in an Army reliability test in November 2007. During the test, the M4 suffered more stoppages than the combined number of jams in the Heckler & Koch XM8; FNH USA’s Special Operations Forces Combat Assault Rifle, or SCAR; and the H&K 416. All three of those weapons use versions of the piston gas system.

I really, really, really don't want to rehash this here, but there are responses to those "tests" out there from various sources. Keep digging.

[rob_s]

The guy said that he shot XXX thousand rounds with no oil. But then he said oil is the key to reliability. I am confused how he ran an AR while dry.

It wasn't "dry", it had "assembly lube" on it. I have three complete BCM rifles in my safe for T&E and each one came with a sheen of oil on the operating parts. It's not like Pannone dunked them all in brake cleaner to strip off every ounce of lube, he just shot the gun as it arrived.

The point here is that with some slight modifications (and the necessity for those is debatable) he was able to go x number of rounds without further cleaning or lubrication. Had he cleaned the gun or added lube he could have gone much, much further as evidenced by some of the high-round-count guns Pat Rogers has that get nothing but more lube from time to time.

I will also say that this obsession that civilian shooters are gaining with high round count failure testing is kind of ridiculous. If you ever find yourself in a situation where your carbine needs to run to that volume of fire without an opportunity to at least lube the gun and maybe run a chamber brush in and out a few times, you are going to be good and well fucked long before you get the chance. Military applications are one thing, but civilian obsession with these things. Seems like every rifle I review now people want to know "how many rounds did it go between cleaning?!"

[silencertalk]

I really, really, really don't want to rehash this here, but there are responses to those "tests" out there from various sources. Keep digging.

First, is the test available for me to see?
Second, what is the short answer to the response?

[silencertalk]

The point here is that with some slight modifications (and the necessity for those is debatable) he was able to go x number of rounds without further cleaning or lubrication. Had he cleaned the gun or added lube he could have gone much, much further as evidenced by some of the high-round-count guns Pat Rogers has that get nothing but more lube from time to time.

I will also say that this obsession that civilian shooters are gaining with high round count failure testing is kind of ridiculous. If you ever find yourself in a situation where your carbine needs to run to that volume of fire without an opportunity to at least lube the gun and maybe run a chamber brush in and out a few times, you are going to be good and well fucked long before you get the chance. Military applications are one thing, but civilian obsession with these things. Seems like every rifle I review now people want to know "how many rounds did it go between cleaning?!"

Can you tell me about the Pat Rogers guns? He just adds oil and no cleaning? How many rounds have they gone? Without a can I assume?

As for obsessions, I want to know I am using a good design. If a Sig P220 can shoot 20,000 rounds MTBF and an under $1000 1911 can shoot 150-300 rounds MTBF, I think people should know that there are handguns that are between 60 and 120 times more reliable than an under $1000 1911.

[-k-]

Dust test info.
http://www.defensereview.com/defrev-rec ... -document/

[silencertalk]

http://www.defensereview.com/defrev-rec ... -document/

http://www.defensereview.com/colt-m4-ca ... lity-test/

Ok. So the SCAR and XM8 were given the chance to cherry pick. No way around that really. If you ask HK for 10 XM8s they are going to ask why you want them.

The 416 did well considering 1 out of the 10 units caused most of the failures. It was just a bad unit and not likely part of the design.

Now we know the 416 is what some would call over-gassed. So I wonder how an M4 would compare to a 416 if you just increased the gas port by 0.005 and used a 416 action spring to help it close?

And they al failed headspace after 600 rounds? Was it the bolt or the barrel extension which wore? Did changing to a new bolt restore headspace to the original specs? They did not make that clear.

[Mongo]

Now we know the 416 is what some would call over-gassed. So I wonder how an M4 would compare to a 416 if you just increased the gas port by 0.005 and used a 416 action spring to help it close?

Robert, I believe that you would also have to increase operating mass of the bolt group as well. When you open up the gas port it will increase the velocities of the op group and cause more malfunctions and premature wear. Increasing the bolt mass will keep the bolt group vel. down giving the case time to spring back and extract more easily. Its a balanced system that you can't monkey with just a few aspects and not have adverse problems else where. The HK 416 essentially increased mass, spring rates, and operating energies to get a more robust primary extraction and cycling. This of course results in the increased recoil. An AK works but it also beats the s--t out of you and is not controllable in F/A except by the best trained users.

[-k-]

We have no idea on round count before the test for the M4s used, extractor springs could have been past service life.