HK416 /DI parts compatibility

AR/M4 AR General Discussion
1

A bit of an FYI for those running HK416/MR556 guns.

One of the biggest complaints about the HK system, or any piston AR for that matter is the lack of parts compatible with DI guns and of course, the expense of proprietary replacements.

Due to an online discussion I was involved in about what was the best gun for long term disasters, DI or piston, where one of the most common shortcomings of Piston systems that was mentioned was lack of spare parts, I decided to find out exactly what was and was not compatible.

As it were, a “full” HK rifle will have less parts compatibility unless some entire sub-groups are swapped out, such as LPKs, the entire lower, etc.

I decided to forgo the headache of not being able to use any mag avalible and just bought an MR556 upper and modded it to fit my needs (reprofiled the barrel, cut to 14.5, perm attached a bi-lock, had the whole thing QPQ nitrided, etc). I also went with a Noveske lower and a Vltor A5 stock system (LMT sopmod for the stock body).

This means that my entire lower is fully compatible with any standard DI gun.

But what about the BCG?

Considering that the carrier itself is considered a major component and is expected to last the life of the firearm, replacement and compatibility is not an issue. The bolt, ejector, extractor, firing pin, etc. are an issue and a possible liability. Especially considering that when spare HK bolts can be found, they are stupidly expensive. Like 5 times the cost of a DI bolt.

Thusly, I decided to do a full swap of a brand new Noveske bolt assembly into the HK carrier and check critical dimensions. Turns out that it fits right in en-mass, and firing pin protrusion is proper and it head-spaced as it should.

But, as someone on another site pointed out, fit doesn’t necessarily translate into function. So I headed to a square range today to do two tests. The first one was to finally shoot this bitch for groups to see how the barrel work affected accuracy (took the profile from 1" under the handguards down to .7"), especially considering that piston guns have a bit of a reputation for less accuracy due to the extra mass moving over, and impacting on the barrel.

After a thouroughly frustrating fucking day trying to get good groups (the scope wouldn’t zero for shit at first. Now I know why they guy gave it away for free) I finally got a 5 shot group (with fuckin’ flyer of course) that I could live with and said “screw it, i’m done for the day”.

I then did some happy blasting to with video to show that the DI bolt was working fine, and then re-attached the Aimpoint/po-boy and re-zeroed. That’s when I nearly shit myself, but in a good way.

Point being is that for all 170 rounds, the DI bolt assembly worked fine.

I shot the entired session with the DI bolt in place and took two videos, one showing install shortly after setting up the target, etc., and another after the last accuracy group that I shot where I was only shooting to demonstrate that the gun being fired was in fact with the AR bolt, as I took the BCG apart after shooting to prove that it is in fact a DI bolt, cam pin, and firing pin(and the HK firing pin spring was retained).

I also took several before and after pictures of the bolt head itself to see how 170 rounds might affect it. My primary concern was for the bottom lugs of the bolt. Since the bolt fits into the HK carrier freely, even though it still has the gas rings, I was expecting that upon the bolt coming in contact with the top round in the magazine, it would be cammed downward as it stripped the top round from the magazine and possibly come into contact with the bottom lugs on the barrel extension, thereby being out of alignment and causing excessive wear and marring of the bolt lug edges.

Turns out I had nothing to worry about and the brand new Noveske bolt barely shows any signs of wear, much less mis-alignement strikes/wear.

The biggest difference I noticed was brass throw. The DI ejector spring is in fact weaker then the HK ejector spring, so brass was ejecting further forward as the bolt was able to travel rearward further before the brass actually struck the deflector. It still ran flawlessly though, and while deflected to a different location then normal, the ejection was still consistent.

Before pics:


After pics:


And here are the videos I shot. First one showing install, second one showing shooting and then removal for proof.

http://smg.photobucket.com/albums/v186/grumpym4/?action=view&current=ARboltinstall.mp4
http://smg.photobucket.com/albums/v186/grumpym4/?action=view&current=shootandprove.mp4

This leaves the only incompatible parts being obviously the barrel/gas block, the piston assembly vs. the gas tube, the carrier, and the firing pin retainin pin which is proprietary in the MR556, but can be easily fabricated.

Being that the barrel, op-rod and piston are also considered major assemblies that should last the life of the gun (especially with the QPQ nitriding), my only possible concern would be the op-rod spring, and the piston gas rings.

I’ll do another test shoot later to see how the gun works without rings in place just for shits and giggles.

2

You’re a Jedi Master Grumpy.

3

I’ll do another test shoot later to see how the gun works without rings in place just for shits and giggles.

It will fire, but fail to cycle, so you will essentially have a bolt action gun, and have to rack the charging handle every time.

Happens with only one gas ring removed. At least it did on the one case I have seen.

4

But this on a piston gun, not DI.

5

Arctic1 is Norse Mil and uses the 416N as part of his job.

I believe what he has to say about it.

I guess the question then is, how long do gas rings last on the 416?

As in how many sets do I need to last the life of the gun?

6

Well, I’m a sample of one, but I have shot 10K+ through my gun, and I have yet to change out any gas rings or the piston itself. The only part replacements done to my gun, is what I listed in the HK416 feedback thread:

https://www.m4carbine.net/showthread.php?t=92876&highlight=hk416&page=5

Post #92

An armorer also swapped out the handguard, because I supposedly over-tightened the handguard screw, and messed up the threads. The manual states the the screw is to be tightened using a torque wrench set to 7N. In my experience that is to loose when doing heavy shooting, and the screw has come loose, and induced wobble in the handguard.

And our armorers don’t replace gas rings; if one or more is broken or missing, they just issue a new piston.

I have only seen one case myself, where one gas ring was gone, in the 4 years my unit has used the guns.

7

But he is talking about removing the gas rings on the bolt, not the piston.

8

I read it like he is going to remove them from the piston.

Why would it matter if the Noveske bolt has gas rings on it or not? There is no gas directed back into the upper reciever or into the bolt carrier. The seal from the gas rings forms in the expansion chamber inside the gasblock, where they are mounted on the piston.

The whole point of the swap was to see if non-OEM parts manufactured for a different system, would work in the HK MR556, in order to see what can be used as spare parts if he is not able to get hold of HK spares for his BCG.

Read the last paragraph again:

This leaves the only incompatible parts being obviously the barrel/gas block, the piston assembly vs. the gas tube, the carrier, and the firing pin retainin pin which is proprietary in the MR556, but can be easily fabricated.

Being that the barrel, op-rod and piston are also considered major assemblies that should last the life of the gun (especially with the QPQ nitriding), my only possible concern would be the op-rod spring, and the piston gas rings.

I’ll do another test shoot later to see how the gun works without rings in place just for shits and giggles.

9

I was talking about removing the rings from the piston, not the bolt. I could care less if there were gas rings or not on the bolt. They have zero to do with the function of this kind of rifle.

Sorry if I left some ambiguity with the initial post.

10

Thanks and excellent post! I have always wondered and been tempted to try and fire mine without the rings but never thought it a good idea.
I have an extra set of rings just in case but could never figure out how to replace them when the time came. Any thoughts?

11

Oh I had assumed that you were talking about the bolt still, as it seemed common sense that the rifle would not cycle with out the piston rings.

12

Maybe, maybe not. Most piston systems have a pretty robust gas system as it is and the HK is no exception.

Part of the system is also a much heavier buffer and several pound stronger action spring.

One of the things i’m going to try on my next test session will be trying a spare piston with no gas rings, and should that fail (as I expect it will), I will then swap lowers to a standard carbine spring and H buffer and see if that does allow it to work.

The reason for this is that the piston has a shoulder that presses against the gas block and provides full closure of the system, and this just might capture enough gas to allow the system to work with a lighter (standard) spring and buffer.

Once again, this is not testing for every day running, but more of a “what if”, should I ever find myself without access to spare parts or the ability to fabricate as necessary.

In other words, just how far can I make this thing work if my only options for re-supply is standard DI parts.

What it pretty much boils down to is just a bunch of fun experiments because I can.

13

Did you ever figure out your MOA on your cut & profiled barrel?

(Forgive me if you’ve already answered that question from me. I’m a little slow sometimes, lol.)

14

Do you intend to continue this experiment over the long term? I am curious if what you said about possible marring of the bolt lugs might occur over something like several thousand rounds. It often takes hundreds of rounds to see carrier tilt damage the buffer tube on other piston systems, so I can see it taking thousands of rounds to truly see if there are any adverse effects from the bolt swap given the strength of the bolt compared to a simple buffer tube. In any case, you are definitely a braver man than I for doing this.

15

I don’t know how to swap out gas rings, sorry.

We just get new pistons when the gas rings break.

I have never been issued a DI AR either, only fired them, so don’t know how to swap the gas rings out on that system either. Probably isn’t too difficult, but I’ll leave it up to the more knowledgeable guys to answer.

16

A questions: Doesn’t the MR556 have an indentation in the bolt to fit a ridge of the chamber? Wouldn’t this cause a problem for the standard AR bolt to fit in the chamber?

This ridge does not exist in the HK416 chamber, nor does the HK416 bolts have the indentation to fit into the ridge. In other words, a spare bolt from an HK416 will not fit in the MR556.

This whole thing is a leftover from the German civilian design of the MR223 for the German civilian market. It serves to make HK416 bolts unusable in MR223, as under German laws that renders the MR as a “match rifle” as opposed to a military rifle or weapon of war or some nonsense like that.

17

The cut out is in the bolt carrier, not the bolt, and the “ledge” runs inside the upper reciever, not inside the barrel extension.

So a spare HK416 bolt would work, an HK416 bolt carrier would not work.

18

Thanks, Arctic.

I knew it was something like that but did not have the documents handy, and my brain is not working well.

Here is what I have in my safe in regards to HK416 spare parts:

19

Here is the final target of my Saturday accuracy grouping test and DI parts compatibility test. It actually has 3 seperate shot groups on it with two different sight systems and this is what lead to me having a giggle-fest as I finally put everything away before hitting the pistol range just to blow off some steam.

Here is the target with the 3 shot groups marked:

Group number one is the last group with the magnified scope. Rounds 1,2,3, and 5, are in the main group (one through the same hole) with shot number 4 being the flyer. I knew I pulled it when it happened and was a bit upset, but cest la vie.

Group number 2, on the other hand, was AFTER I removed the magnified scope and replaced my Irons and the Aimpoint PRO and was re-verifying zero of my setup with Hornady 75 grain TAP (my go-to ammo). Group 2 is 7 rounds with a 2MOA aimpoint pro and a 2.5x po-boy magnifier. This was with the same point of aim as group 1. Only difference being that Group 1 was shot with Federal 55 grain M193, and Groups 2 and 3 were shot with the Hornady 75 grain TAP.

Group three is after I raised the shot group a little to match the shot group to the offset at 100 yards that would be experienced with a 50 yard zero. Group 3 consists of 3 rounds.

All groups were shot at 100 yards off of of a bench using sandbags to support the rifle at the Poulsbo Sportsmans Club in Poulsbo WA. Keep in mind that I also hand-lapped my barrel prior to nitriding, which will effect how accurate a barrel is (makes it more accurate then stock).

Here are pics of the groups with a tape measure for reference.

Photo 1: The full size, with flyer of Group 1.

Photo 2: the main cluster of Group 1 without the flyer.

Photo 3: full length of group 2 as done with Aimpoint and 2.5x magnifier.

Photo 4: full length of Group 3 after adjusting for 50 yard zero to 100 yard zero match.

Photo 5: screwing around with the little tiny silouhette on the upper right corner of the target with the Aimpoint. Approx 1 round per 2 seconds, 10 shots. I was trying to aim at the bottom of the mini-silouhette, but there wasn’t a lot of contrast in the bright sunlight so he washed out a bit. Compared to shooting at the shoot-n-see targets with the aimpoint which was very easy because all I had to do is move the dot over the black and when two red dots became one, I squeezed the trigger. Easy-peasy.

I have no idea what the equivilent distance would be considering the little guys size, but I think that 7 shots out of 10 isn’t too terribly bad. Of course this doesn’t really accuratly reflect the reality of making such a long shot as there will be a hell of a lot more things effecting a bullet on such a long flight, but it’s fun to do anyway.

As to why I did “better” with the aimpoint, I think there were several factors. I think some has to do with my issues getting crosshairs on magnified scopes to focus due to my astigmatism, and I think I was a bit tense (since I knew this was going up on the 'net, I may have had “performance anxiety” or something), or maybe it had to do with a POS, cheapo 3-9x bushnell scope that was given to me for free. When I removed the shit-tastic magnified scope and relaxed, things just fell into place.

I have no idea why but I find it easier to focus a red dot then crosshairs. Matter of fact, if I look through a magnified scope long enough, the crosshairs actually dissapear.

As to the capabilities of the rifle, I’ll let somebody else figure the math on that.

20

In the case of the bolt vs. bolt lug, it would show up almost immediatly. That is a high impact junction and if the bolt were off axis enough for it to be an issue at all, it would show within a round or two even.

On the other hand, if there are any excess (as in more then there already is in a standard DI system) off axis forces being applied to lets’ say, the bolt cam pin and the bolt upon the unlocking and rearward travel phase of the cycle, that would show in a few hundered or even a few thousand rounds, that is if it didn’t shear a bolt lug or two off before then.

An inspection of the Noveske bolt showed typical wear associated with 170 rounds of ammunition fired. A little polishing of the parkerizing is all. Wear was even on the rear of all of the lugs as well. I think that one of the first places that excess wear caused by off axis bolt/carrier/extension issues would show would be the raised ring on the bolt just forward of the cam pin hole. Wear seemed normal there as well, about the same as the rear of the lugs.

On closer inspection, the forward face of the bolt lugs seem to have more wear on the two lugs in line with the top of the cam pin. A quick re-installation of the DI bolt into the HK carrier confirms that the bolt pivots somewhat on the cam pin during the chambering process and those two lugs are coming in contact with the breech-face before the others.

To be honest, i’m not too terribly worried about that wear as that isn’t a high stress part of the cycle on the bolt and won’t lead to headspace changes, or stressing of the bolt to the point of failiure at any point in it’s life.

The wear i’m concerned about the most is on the rear of the bolt lugs (if the bolt is off axis during the unlocking and extraction phase of cycle).

So far wear on the rear lugs is symmetrical and even.

So far so good.