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Discussion Starter #1
On the 416, not the MR556. Saw it while cleaning the other day and was wondering what it's function is.
 

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Prevents it from blowing the extractor out , thereby keeping the rifle operational if it blows a primer or has a case head seperation.
 

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Since we have yet to see (at least I havn't) a 416 or MR have this issue (blown primer or case head seperation), I suppose we don't know exactly how well the idea works for those issues. Simple logic dictates though, that this midification has a high probability of working as advertised.

That being said, I think it's theoretically one of the better upgrades for the AR that has ever been invented. While some parts of the 416 and its bretheren leave me scratching my head, it's these little nuggets of pure genius such as the extractor lug make it all worth it.
 

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Discussion Starter #6
Since we have yet to see (at least I havn't) a 416 or MR have this issue (blown primer or case head seperation), I suppose we don't know exactly how well the idea works for those issues. Simple logic dictates though, that this midification has a high probability of working as advertised.

That being said, I think it's theoretically one of the better upgrades for the AR that has ever been invented. While some parts of the 416 and its bretheren leave me scratching my head, it's these little nuggets of pure genius such as the extractor lug make it all worth it.
IIRC the biggest "failure" (most common and most deadlining) in the AR family is the failure to extract. You know the old "broken down weapon and cleaning rod in their hand while killed by the NVA" thing (NOT making light of it mind you). So no donuts or D-rings needed for the 416 extractor?
 

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IIRC the biggest "failure" (most common and most deadlining) in the AR family is the failure to extract. You know the old "broken down weapon and cleaning rod in their hand while killed by the NVA" thing (NOT making light of it mind you). So no donuts or D-rings needed for the 416 extractor?
This nub wouldn't help with the typical extraction issues of the AR series. It's merely a last stop measure for the issues initially stated such as blown primer or case head seperation.

The typical extraction issues on the AR platform come from what is called "extractor lift". THis was initially thought to ahve been caused by the speed of the rotating bolt and centrifugal force, but testing proved otherwise. There is a nice little Mil powerpoint presentation on extractor lift and some of the tests the Mil performed to try and figure out the issue.

The location and size of the extractor nub in the receiver extension mean that it is not a factor in helping with extraction issues and if they did try to place a nub in such a location that it was, the rifle wouldn't be able to chamber a round in the first place.

If you're interested, here's a link to the power point presentation regarding extractor lift: www.dtic.mil/ndia/2003smallarms/din.ppt
 

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Extractor nub in HK barrel extension is one of features from OTB (Over The Beach) package in HK416. It supports extractors and prevents it from being blown from bolt due to excessive pressure in barrel when shooting from water, with water in barrel or with obstructed barrel. This prevents catastrophic pressure build up inside upper receiver and provides ability to function after such event. It does not have any function in normal rifle operation.

Here is photo of nub in my old MR223 (new one has one as well):

 

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Discussion Starter #9
This nub wouldn't help with the typical extraction issues of the AR series. It's merely a last stop measure for the issues initially stated such as blown primer or case head seperation.

The typical extraction issues on the AR platform come from what is called "extractor lift". THis was initially thought to ahve been caused by the speed of the rotating bolt and centrifugal force, but testing proved otherwise. There is a nice little Mil powerpoint presentation on extractor lift and some of the tests the Mil performed to try and figure out the issue.

The location and size of the extractor nub in the receiver extension mean that it is not a factor in helping with extraction issues and if they did try to place a nub in such a location that it was, the rifle wouldn't be able to chamber a round in the first place.

If you're interested, here's a link to the power point presentation regarding extractor lift: www.dtic.mil/ndia/2003smallarms/din.ppt
Awesome link! Thanks. Interesting data. While not mentioned in the PP, chrome lining the chamber as well as not using an excess of calcium carbonate in the powder also helps. :wink: Add that to a tropical/humid environment with a "self cleaning" weapon and it's a recipe for disaster.
 

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This prevents catastrophic pressure build up inside upper receiver and provides ability to function after such event.
Please explain that statement.

And for what it's worth, the MR556s (US release) also have the nub. I know yours is the European model (MR223), so I don't know which exact differences HK chose to have between the two models.
 

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Please explain that statement.
I had in mind that rifle with OTB package, will function after firing with water in barrel, because reinforced (in extractor area) bolt will not allow pressure into receiver. This pressure can (and usually do) blow up upper receiver on AR15s. There are materials from HK (video) as well as matter was deeply covered by G3Kurz.

We had topic here on this feature and OTB package, that you probably missed. It was established that while MR223 and MR556 does have all "parts" of OTB package, those rifles are not tested/provided by HK as OTB capable. However my MR223 survived twice events that blown up other rifles (DI and piston ARs of different make), one due to faulty batch of ammo (overpressure) from Fiocchi (also my friend's heavy barreled Steyr AUG survived this ammo with no noticeable damage) and other one that most probably was squib shoot out by second round

Differences between MR223A1 and MR556A1 are different barrel thread (MR223A1 uses M15x1 RH and is delivered with thread protector only) and relocated rear receiver pin. MR223 of older production had several differences from A1 model and also smaller differences in between production batches.
 

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I had in mind that rifle with OTB package, will function after firing with water in barrel, because reinforced (in extractor area) bolt will not allow pressure into receiver. This pressure can (and usually do) blow up upper receiver on AR15s. There are materials from HK (video) as well as matter was deeply covered by G3Kurz.
Exactly how does this nub prevent "pressure into the receiver"?

The excess pressure that destroys an AR receiver is delivered via the gas tube to the BCG and the BCG is destroyed as well as the upper receiver. With the lack of the piston system in the bolt carrier group, this isn't an issue at all with the 416 family of weapons, hence the only pressure that could be applied to the upper receiver is pressure that is directly released from the chamber.

We had topic here on this feature and OTB package, that you probably missed. It was established that while MR223 and MR556 does have all "parts" of OTB package, those rifles are not tested/provided by HK as OTB capable. However my MR223 survived twice events that blown up other rifles (DI and piston ARs of different make), one due to faulty batch of ammo (overpressure) from Fiocchi (also my friend's heavy barreled Steyr AUG survived this ammo with no noticeable damage) and other one that most probably was squib shoot out by second round
I have read this topic as well as many others regarding the 416 family of rifles. I understand how the nub protected the extractor in the instances you describe, but I would really like to know the technicalities of your statement. HK can make any statement they like, as can you. I'm asking for the technicalities that prove this.

It would seem to me that the nub has one of two possible functions:

1) holding the extractor in place to prevent it from flexing or bending during an overpressure event and ensuring that the rifle will still unlock, cycle, and continue to work, which would still result in any remaining pressure to blow back into the receiver thusly invalidating this statement due to the fact that the rifle is still functioning as intended, or

2) hold the extractor in place to prevent if from flexing or bending during an overpressure event while at the same time retarding the unlocking of the bolt and cycling of the BCG until pressures have dropped to a safe level, which would in turn lock the system in place until after the piston has attempted to engage the BCG and failed, thusly preventing the system from cycling at all.

Which one is it? When your MR possibly shot the lodged bullet out of the barrel, did it cycle the rifle and did you find a fresh, live round in the chamber? If so, then the nub doesn't help restrict excess pressure in any manner except on the extractor. This is a product of simple physics.

Differences between MR223A1 and MR556A1 are different barrel thread (MR223A1 uses M15x1 RH and is delivered with thread protector only) and relocated rear receiver pin. MR223 of older production had several differences from A1 model and also smaller differences in between production batches.
I remembered how worried we were that HK was going to release the MR series here in the states with the offset takedown pins. I'm very glad that isn't the case. Also, are there are no chamber dimension variances between the two?
 

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Exactly how does this nub prevent "pressure into the receiver"?

The excess pressure that destroys an AR receiver is delivered via the gas tube to the BCG and the BCG is destroyed as well as the upper receiver. With the lack of the piston system in the bolt carrier group, this isn't an issue at all with the 416 family of weapons, hence the only pressure that could be applied to the upper receiver is pressure that is directly released from the chamber.
But this nub is in piston HK and it is only part of package. There is no pressure going trough gas tube in HK, so it is not concern.

It would seem to me that the nub has one of two possible functions:

1) holding the extractor in place to prevent it from flexing or bending during an overpressure event and ensuring that the rifle will still unlock, cycle, and continue to work, which would still result in any remaining pressure to blow back into the receiver thusly invalidating this statement due to the fact that the rifle is still functioning as intended, or

2) hold the extractor in place to prevent if from flexing or bending during an overpressure event while at the same time retarding the unlocking of the bolt and cycling of the BCG until pressures have dropped to a safe level, which would in turn lock the system in place until after the piston has attempted to engage the BCG and failed, thusly preventing the system from cycling at all.
From what I know, normally AR15 extractor does not support case in this point and in case of overpressure it gives back and allows case rupture in extractor area allowing pressure into chamber in initial phase of cycle - when pressure in its peak. Nub prevents extractor from giving up. Thicker CHF barrel at same time is able to stand this pressure (other part of OTB package). When gun bolt unlocks pressure is already dropping. It is for sure high, but HK tests shows that is not high enough to damage rifle. Of course it still can only live trough "so much pressure", but I do not know how much it is and I do not intend to test it.

Which one is it? When your MR possibly shot the lodged bullet out of the barrel, did it cycle the rifle and did you find a fresh, live round in the chamber? If so, then the nub doesn't help restrict excess pressure in any manner except on the extractor. This is a product of simple physics.
It did cycle very violently, like it would be some juicy round, not puny 5.56. Firing pin retaining pin bend and break on one leg from it (MR223 used to have standard AR pin).


I remembered how worried we were that HK was going to release the MR series here in the states with the offset takedown pins. I'm very glad that isn't the case. Also, are there are no chamber dimension variances between the two?
HK MR223 is marked as ".223rem" but chamber is CIP (HK must use CIP by law). It so happens that CIP has same chambers and same pressures for .223rem and 5.56 NATO. Same was with SL-8. HK marks civilian rifles as .223rem or .308win for Political Correctness sake in Germany. If barrel blanks for MR556 from HK are made with chambers, then those are CIP .223/5.56 chambers. BTW CIP uses different pressure test procedures than SAAMI, so top pressures set in those standards can not be directly compared.
 

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But this nub is in piston HK and it is only part of package. There is no pressure going trough gas tube in HK, so it is not concern.
I'm aware of this. That's why I said "this isn't an issue at all with the 416 family of weapons,"

From what I know, normally AR15 extractor does not support case in this point and in case of overpressure it gives back and allows case rupture in extractor area allowing pressure into chamber in initial phase of cycle - when pressure in its peak. Nub prevents extractor from giving up. Thicker CHF barrel at same time is able to stand this pressure (other part of OTB package). When gun bolt unlocks pressure is already dropping. It is for sure high, but HK tests shows that is not high enough to damage rifle. Of course it still can only live trough "so much pressure", but I do not know how much it is and I do not intend to test it.
Interesting that HK would claim this. Every ruptured case I have ever seen happens just forward of where the extractor sits and there is nothing the extractor can do to stop it. The extractor is actually resting on the thickest part of the casing to begin with. I can see how the nub prevents the extractor from actually being bent or destroyed by the escaping gasses of a ruptured case, but the act of holding the extractor in place does nothing to contain the escaping gasses.



It did cycle very violently, like it would be some juicy round, not puny 5.56. Firing pin retaining pin bend and break on one leg from it (MR223 used to have standard AR pin).
Then this holds with scenario number 1 that I gave. All the nub does is prevent the extractor from flexing, bending, or breaking, but doesn't actually do anything to contain pressure as HK claims. Silly HK. To put it bluntly, if a caseing is going to rupture, break, whatever, it's going to do it regardless of how reinforced the extractor is and all the gun can do is be designed to mitigate the affects. This nub is one of those mitigating factors and a good one at that, but yet it still doesn't do everything that it's claimed to do.

It's still a good idea in my book, a VERY good idea considereing how well your MR223 came through the incident.




HK MR223 is marked as ".223rem" but chamber is CIP (HK must use CIP by law). It so happens that CIP has same chambers and same pressures for .223rem and 5.56 NATO. Same was with SL-8. HK marks civilian rifles as .223rem or .308win for Political Correctness sake in Germany. If barrel blanks for MR556 from HK are made with chambers, then those are CIP .223/5.56 chambers. BTW CIP uses different pressure test procedures than SAAMI, so top pressures set in those standards can not be directly compared.
 

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This nub is one of those mitigating factors and a good one at that, but yet it still doesn't do everything that it's claimed to do.
As I wrote, it can take only "so much pressure". There is always a point when things break. HK OTB package just moves this point step or two up.
 

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As I wrote, it can take only "so much pressure". There is always a point when things break. HK OTB package just moves this point step or two up.
As Montrala has explained the "nub" or limiting pin if you will was added to the HK416 based on the need for the weapon to survive the Over-the-Beach (OTB) test that the USSOCOM does on select individual weapons for those who operate out of the surf/water. The FN SCAR Light (MK16), HK G36 along pass this test. The video at this link below shows what happens when a 5.56mm carbine without the OTB rating has a bore obstructed by water and/or a trapped air bubble. The damage comes not from the gas in the gas tube (though the tube can rupture) but the capillary effect of the water in the small 5.56mm bore. That extreme overpressure blows the case, fractures the bolt head and destroys the receiver and maybe the shooter also. The limiting pin prevents over pivot of the extractor and this damage to that component.

HK first added this to the HK416 and a standard feature and thus to some of the commercial variants to avoid having to make special parts that is why some have it and others don't based on when they were made and which parts were on hand in the factory at the time. The true and complete OTB-rated guns like that one in the video also have blow-out vents in the bolt carrier, buffer tube and stock for water drainage. OTO-rated HK416's no only survive this test where the Diemaco C8 (Canadian M4) in the video did not after only one round fired, the OTH HK416 survived 20+ tests one after the other and functioned normally afterwards firing all rounds in teh magazine.

This is a major accomplishment that is posible not only on the OTB features but also the materials used by HK in the barrel, bolt head and to a far lesser degree the piston rod operating system.

BTW - The man in the video is the famous HK demonstrator Robert Hirt who now works for SIG Sauer. Wanna bet the SIG 516 is well tested in this regard?

DO NOT EVER try this test at home as it is extremely dangerous. Countless folks have died with obstructed bore occurances in AR platforms.

m4 verus HK416 - Bing Videos

G3Kurz
 

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I heard the G36 has blowout vents in the barrel. Does the G36 have blowout vents in the bolt carrier like the OTB HK416 or is that not required? Does the Mk16 still have 2 second drain time or is it zero drain time now? A powerpoint presentation by Troy Smith a couple of years ago said the CQB version of the MK16 has 0 drain time while the other version have 2 second drain time. The current brochure by FNH USA says the Mk16 has 0 drain time.
 

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I heard the G36 has blowout vents in the barrel. Does the G36 have blowout vents in the bolt carrier like the OTB HK416 or is that not required? Does the Mk16 still have 2 second drain time or is it zero drain time now? A powerpoint presentation by Troy Smith a couple of years ago said the CQB version of the MK16 has 0 drain time while the other version have 2 second drain time. The current brochure by FNH USA says the Mk16 has 0 drain time.
The "blow out" vents in the G36 are not the same as OTB vents for safe function out of the water. In the G36 they are there in the event of an obstructed bore occurance or ruptured case to protect the user. The MK16 does have 0 seconds drain time or it is not really OTB rated. The M4 needs 2-3 seconds drain time and the chamber being slightly opened to be fired safe and that is a decade if the operator needs to fire immediatey after surfacing.
G3Kurz
 

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You know the old "broken down weapon and cleaning rod in their hand while killed by the NVA" thing (NOT making light of it mind you).
Exactly, "OLD" problem that has long been resolved. That problem was due to the military trying to save money by fielding a weapon without the full weapon "system" as tested. Why do people still hold this against the AR?
 
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