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Discussion Starter · #1 · (Edited)
So, a while ago I bought a 10.4 MR556 upper (done by IGF, uses the factory 16 inch gas block) and it would only cycle XM193 properly. It would not cycle PMC Gold nor Wolf Gold until I used a grub screw (with no hole) in the front of the gas block (where it was tapped).

Fast forward to last week. I saw a 10.4 (HK Parts) Gas Block for sale on here and figured I'd buy it and attempt to fix the problem. Still won't cycle the PMC Gold or the Wolf Gold as it is undergassed.

I got the block off of one of these setups.
https://www.hkparts.net/shop/pc/viewPrd.asp?idproduct=17824&idcategory=132

Questions:
1. What is the difference between the 10.4 and 16 inch gas block?
2. What am I doing wrong?
 

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So, a while ago I bought a 10.4 MR556 upper (done by IGF, uses the factory 16 inch gas block) and it would only cycle XM193 properly. It would not cycle PMC Gold nor Wolf Gold until I used a grub screw (with no hole) in the front of the gas block (where it was tapped).

Fast forward to last week. I saw a 10.4 Gas Block for sale on here and figured I'd buy it and attempt to fix the problem. Still won't cycle the PMC Gold or the Wolf Gold as it is undergassed.

Questions:
1. What is the difference between the 10.4 and 16 inch gas block?
2. What am I doing wrong?
Factory 10.4" gas block is not vented. Use the forum Search and you'll find plenty of info about it. Some later models were vented but to a very specific diameter.

Unless suppressed, no need for vent on 10.4.


Sent from my iPhone using Tapatalk
 

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I have a MR556 cut down to 10.4" and using a 416 10" gas block. (no vent) It runs like a champ. The only ammo I have ever used is Hornady .223 55gr FMJ. Gun Firearm Trigger Assault rifle Airsoft
 

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The Gas port size on the barrel is the problem!! Now, I am NOT a gunsmith, an Armorer, or any kind of expert. I am lucky to have very close friends whom are.

Having different size Gas Ports on the barrels is how HK gets around having different length op rods or gas blocks in different locations.

Any gunsmiths out there feel free to correct me if I am incorrect, but IIRC this is the topic that came up with my buddy about mixing and matching bbls and GBs on the HK 416 or MR barrels.
 

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Discussion Starter · #6 ·
The Gas port size on the barrel is the problem!! Now, I am NOT a gunsmith, an Armorer, or any kind of expert. I am lucky to have very close friends whom are.

Having different size Gas Ports on the barrels is how HK gets around having different length op rods or gas blocks in different locations.

Any gunsmiths out there feel free to correct me if I am incorrect, but IIRC this is the topic that came up with my buddy about mixing and matching bbls and GBs on the HK 416 or MR barrels.
That would be too simple an answer! Anyone know the gas port sizes on a 16 vs a 10.4?
 

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My information contradicts Jnap's, but let me first ask this:

Are you SURE the gas block you have currently is indeed a 10.4" gas block? It should be marked as such.
 

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Well I don't think Bob has a HK German factory block but instead the US made one based on the link he posted...
Ah, I see that now

What Jnap says would be true...for a DI gun.

To my recollection, the standard 416's have the same gas port size in the barrel. The differences were in the gas block itself in that the longer barrels had a bleed off valve and the early 10.4's didn't. Due to some issues, later 10.4's started showing up WITH bleed off valves. However, the bleed off valve port was SMALLER than that of it's longer barreled variants. This is why there are STILL different gas blocks for the respective barrel lengths despite the presence of a bleed off valve port. The only time I ever remember hearing about variations in gas port size in class was in reference to the IAR cyclic rate, but that was a long time ago.

Since you are running an aftermarket, it's very likely something is not true to spec and/or a failure to seal. I would be willing to bet that the HKparts guns have an oversized gas port both in the block and barrel as well so they will run (like DD's commercial MK18's). Couple this with the fact that you are running notoriously weak .223 compared to a full-blooded 5.56 chambering, I'm not overly surprised to hear of the issues.
 

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That gas block is US made sold by Adam over at HKParts, its not a real true 416 10" gas block. Also port size on the barrel isn't the most important issue as its the port size on the gas block itself that matters and that varies between 416 gas blocks.
 

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Are you running the HK buffer and HK spring?
I was wondering and about to ask the same....I have seen lots of info on here regarding using Wolff spring and a specific buffer which I can't remember exactly.....that might have been strictly for suppressed and unsuppressed perfection though
 

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That gas block is US made sold by Adam over at HKParts, its not a real true 416 10" gas block. Also port size on the barrel isn't the most important issue as its the port size on the gas block itself that matters and that varies between 416 gas blocks.
So this is a non HK Gas Block, Barrel and Upper. How could anyone know anything about that except the people who build the parts, as it has no relevance with what HK produces or sells?!
 

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Discussion Starter · #17 ·
Yeah, I'm realizing that. Too many variables. I'm gonna go back to my old setup.
 

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I was wondering and about to ask the same....I have seen lots of info on here regarding using Wolff spring and a specific buffer which I can't remember exactly.....that might have been strictly for suppressed and unsuppressed perfection though
Most went with the following:

Vltor A5
Heavy Buffer(10oz in mine)
Wolff XP Rifle spring

Works with 55gr/62gr on a 10.4" upper with a factory plugged HK block(came with a real deal 10.4" 416 barrel) both suppressed and unsuppressed.
 

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So this is a non HK Gas Block, Barrel and Upper. How could anyone know anything about that except the people who build the parts, as it has no relevance with what HK produces or sells?!
If you look on HKParts.net pay close attention to the origins of manufacture some say Germany and some say US. They make a 10" barrel and gas block combo and also sell the parts individually as well. The true 416 parts will say Germany if you look at the item description.
 

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Speaking of HKParts, they are now listing HK416 10.4 vented gas blocks for sale. They list them as HK OEM parts, not as HK "style" parts like the barrel kit they also sell.
HK416 10 Gas Block
This may solve the OP's cycling problems.
 
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