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Discussion Starter · #1 · (Edited)
I am having some issues with my HK MR556 14.5 barrel conversion kit that I recently purchased from HKparts.net. Any help would be appreciated.

I am having failure to feed issues. I can only get one to two shots off at a time. After every shot, I have to rack it before I can get the next shot off. As you can understand this is very frustrating.

To start, I have been using USGI OK mags with magpul followers, PMAG Gen 3s and HK maritime steel mags. I have been having the same failure to feed issue with all the mags.

As for ammo, I tested it with 55 grain and 77 grain ammo. Again all with the same issue. The same ammo has been working great in all my DI AR15s.

As for the Gas Block, the gas block that came with the kit has a stamped 14 on it. So I am assuming that it is the proper gas block and gas ports for my 14.5" barrel.

From what I can tell, the barrel has been installed correctly. I do have a geissele HK mr556 trigger installed along with a Surefire muzzle brake.

Lastly, I am not shooting this suppressed.

If anyone has heard of this or has experienced the same issues. Your knowledge would be appreciated

Thank you
 

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Sounds like its undergassed, no experience with HKparts conversion but they may have undersized the gas port a bit. What spring/buffet combo are you running and have you tried a lighter buffer?

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Which part of the cycle of operation is the malfunction occuring?`
What does the malfunction look like?

Does it eject the spent brass, but does not feed and chamber a new round?
Then it is short stroking, which is a gas issue.
 

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Buffer and spring??
 

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Buffer and spring??
What ^^^^^ said. Use the HK416 buffer/spring....if it is HK, change it out with a new one. That's what it "sounds" like.
Out of the 15-20 uppers I've got.....I can honestly say that I've never had a FTF/FTE.....and I probably go a few thousand rounds each year.
Makes a Rolex look like a Ford Pinto. :biggrin:
 

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What ^^^^^ said. Use the HK416 buffer/spring....if it is HK, change it out with a new one. That's what it "sounds" like.
Out of the 15-20 uppers I've got.....I can honestly say that I've never had a FTF/FTE.....and I probably go a few thousand rounds each year.
Makes a Rolex look like a Ford Pinto. :biggrin:
He’s not the first to report issues with HKParts conversion kits. Your HK OEM uppers I’m sure work flawlesslesly just like mine.

My guess, its the Quality of the conversion kit. If OP has his original barrel, I’d say swap the gas blocks as the HK 14.5” and 16” (and I think 20” 416) gas blocks are all the same. See if that solves the issue.

NB, if the barrel gas port isn’t HK spec, using an HK OEM gas block won’t solve the issue.

Remember these are Todd Bailey products, if I’m not mistaken. Caveat Emptor.
 

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Discussion Starter · #7 ·
Thanks for all the response's. Ill try to give you more detail to the issues I am having with my rifle. Hopefully it can answer some of the stuff you guys have brought up and maybe shed some new light on the issue.

To start, I bought a factory complete HK MR556 with that heavy 16.5" barrel. It worked flawlessly right out of the box. I wanted to go down to a 14.5" so it would feel a little more balanced up front. HKparts.net has a "conversion kit" which includes a 14.5" barrel with flash hider, gas block, barrel nut and wrench. All I had to do was take off my OEM 16.5" barrel assembly bolt on the new hkparts.net 14.5" setup and use my OEM piston assembly. I thought it would be that easy, since I have built plenty of DI AR's from the ground up. I guess it wasn't going to be that simple.

As for the buffer spring/buffer I am using... I am using the OEM HK buffer spring and OEM HK buffer with the red dot that came with my factory MR556.

To be more specific on the malfunction... Its main symptoms is a failure to feed a new round into the chamber and failure for the bolt to lock open after that last round is shot. It has no problems ejecting the spent brass.

I tried swapping my HK OEM gas block with the HKparts.net kit gas block to see if that would fix my issues. Unfortunately the HK OEM gas block would not fit on the HKpart.net 14.5" barrel. No matter what I would do to try and fit it. The OEM gas black was just to small for the HKparts 14.5 conversion barrel.

From reading your guys suggestions. I am starting to think I have an under gassed setup due to sub quality parts from HKparts.net 14.5 "conversion kit". I will contact HKparts.net to see if they can help with my issue.

Thank you
 

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It may not be under-gassed; it could also be over-gassed.

In certain early suppressed guns (I know yours isn't, but...), the lack of bleed-off valve meant a heavily gassed gun that the magazines could not keep up with.

Look at your ejection pattern and try the limp-wrist test. Hold the gun out 2-handed like you would a pistol and fire it. DI builders tuning AR's do this to ensure the gun has enough juice to cycle under less-than-ideal conditions. If you do this and it starts to work, you're over-gassed. If your brass dribbles out/stove-pipes, it's under-gassed.

More often than not, commercial guys (and likely this conversion kit) will err on the side of over-gassing to compensate for people's use of under-powered .223 junk ammo so they don't have to hear people's complaints (like the DD Mk18's initial cavernous gas port).

Either way, that conversion is sub-par POS and the vendor should fix/replace it. Were it me, I'd just have your 16" chopped or pick up one of the factory 14.5" barrels Gatewood supply has in stock. The barrel is the heart of a rifle...this has all the sensibility of dropping a lawn mower engine into a Porsche. I'm no fan of the MR556 barrel, those aftermarket kits are just cruel (even more so if RegularGuy is correct). Good Luck
 

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Discussion Starter · #9 ·
It may not be under-gassed; it could also be over-gassed.

In certain early suppressed guns (I know yours isn't, but...), the lack of bleed-off valve meant a heavily gassed gun that the magazines could not keep up with.

Look at your ejection pattern and try the limp-wrist test. Hold the gun out 2-handed like you would a pistol and fire it. DI builders tuning AR's do this to ensure the gun has enough juice to cycle under less-than-ideal conditions. If you do this and it starts to work, you're over-gassed. If your brass dribbles out/stove-pipes, it's under-gassed.

More often than not, commercial guys (and likely this conversion kit) will err on the side of over-gassing to compensate for people's use of under-powered .223 junk ammo so they don't have to hear people's complaints (like the DD Mk18's initial cavernous gas port).

Either way, that conversion is sub-par POS and the vendor should fix/replace it. Were it me, I'd just have your 16" chopped or pick up one of the factory 14.5" barrels Gatewood supply has in stock. The barrel is the heart of a rifle...this has all the sensibility of dropping a lawn mower engine into a Porsche. I'm no fan of the MR556 barrel, those aftermarket kits are just cruel (even more so if RegularGuy is correct). Good Luck

Thanks for the tip on the limp-wrist test. I tested my rifle yesterday using the limp-wrist test and discovered the hkparts.net conversion kit is over-gassed. During the test I was able to shoot 28 rounds in a row with no issues. However, on the final round of the test, I did not get the bolt to lock open. On my next mag, I shouldered the rifle again, I had a failure to feed malfunction on every round. This conversion kit will be taken off my rifle immediately. I will look at gatewood supplies factory barrel and compare costs to having IGF work on my OEM 16.5" barrel.

Thanks for the advice. I now have a clear description of my conversions kit issues, that I can explain to HKparts.net. I will see how they handle it soon.
 

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Thanks for the tip on the limp-wrist test. I tested my rifle yesterday using the limp-wrist test and discovered the hkparts.net conversion kit is over-gassed. During the test I was able to shoot 28 rounds in a row with no issues. However, on the final round of the test, I did not get the bolt to lock open. On my next mag, I shouldered the rifle again, I had a failure to feed malfunction on every round. This conversion kit will be taken off my rifle immediately. I will look at gatewood supplies factory barrel and compare costs to having IGF work on my OEM 16.5" barrel.

Thanks for the advice. I now have a clear description of my conversions kit issues, that I can explain to HKparts.net. I will see how they handle it soon.
By the time you do all the shorten/reprofile/nitride voodoo to the MR, you're about $600ish shy of what the Gatewood (listed as NEW) barrel would run. The barrel is arguably the most critical part of the 416 system so...how much is it worth to you to have a sure-thing?
 
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