HK416 upper with AR-15 DI internals?

[scdrnr]

Hi, this is my first post here.
I'm slowly building my first AR-15 piece by piece. Unfortunately I was sent an HK416 upper instead of the regular AR-15 upper that I ordered and since I live in a country where parts are both expensive and hard to come by I can not just return it so if it's possible I want to include it in my build instead of going through all the hassle aquiring a new one.

I'm aware the HK416 uses a piston and rod whereas the AR-15 uses direct impingement. I haven't managed to get hold of any blueprints or drawings with measurements of the AR-15 upper so basically I've been restricted to studying pictures and comparing with the HK416 upper...

The differences I've noticed so far is:
* The HK416 rail sits a bit taller
* The bulge on the forward left side is more rounded than on the AR-15 upper
* Longer treading towards the barrel, metric 1.5mm treads
* The hole on the front face where the piston rod is supposed to sit is shaped different and has a treaded removable bushing

I have a standard M16/M4 full auto bolt carrier which slides in nice + mil-spec full auto trigger group and an M16A1 lower. Upper and lower mates well and from what I've read the trigger group should also work fine.

A little help needed here!

1.) Will my HK416 upper work with regular DI AR-15 internals if I machine the piston rod hole to fit a standard gas tube instead?
2.) Could someone please measure the diameter of the x-shaped hole in the front of an AR-15 upper or provide drawings with measurements of it?
3.) Will I experience any other problems, except for the metric treads on the barrel connection? I'm mostly thinking about the extra length of the HK416 upper barrel extension... Are there readily available "conversion nuts" or do I have to have one made?

I tried to capture as much details as possible. Please have a look at the pictures below and please advise!



Kind regards
/scdrnr

[TMS951]

If you put the bolt carrier in the upper does the gas key line up with the hole in the upper?

You will need an HK416 specific rail.

[scdrnr]

It lines up perfectly, fully centered.
The pictures failed to attach. I'm trying to post them again.
https://picoolio.net/images/2020/01/29/HK416_upper01fb059205f98f65cf.jpg
https://picoolio.net/images/2020/01/29/HK416_upper028f7f2c3fd791f257.jpg
https://picoolio.net/images/2020/01/29/HK416_upper0373f54175847c7018.jpg
https://picoolio.net/images/2020/01/29/HK416_upper04e6fa3239da39cf71.jpg
https://picoolio.net/images/2020/01/29/HK416_upper050e88b5423493ecbd.jpg
https://picoolio.net/images/2020/01/29/HK416_upper0662621588ad28fc07.jpg
https://picoolio.net/images/2020/01/29/HK416_upper079bc0aae6638aaddc.jpg
https://picoolio.net/images/2020/01/29/HK416_upper08f4cb2f1d7339ed5b.jpg
https://picoolio.net/images/2020/01/29/HK416_upper09e659d3b1df56cb80.jpg
https://picoolio.net/images/2020/01/29/HK416_upper10678cdff28e080070.jpg
https://picoolio.net/images/2020/01/29/AR-15_bolt_carrier0197c8faa76ceb82c3.jpg
https://picoolio.net/images/2020/01/29/AR-15_bolt_carrier02e49f659213d3ec2f.jpg

[RMike89]

Just curious, what part of the world do you reside in?

Honestly, the cost of hk416 rail/barrel/gas jet alone makes your endeavor an expensive one my friend. My $.02 is to just eat the cost of the upper, get yourself a mil-spec upper and go from there before you go further down the 416 piston -> DI conversion rabbit hole IF those parts are obtainable, barring any difficulties with your country's laws & regulations.

If those parts are not easy to come by, totally subbed to this thread because the outcome, if you're successful, will be radical.

Also welcome!

[El Vaquero]

Knowing H&k I wouldn’t be surprised if they did something to the upper receiver to make it not doable. But I’m with ya on what you’re trying to do. I have an old piston LWRC where I’ve taken it apart to look at. On my LWRC it looks doable as it appears to be a standard upper receiver with only a bushing added where the gas tube would go where there is now a piston. Similar to your 416.

I love me some H&k but they rarely make anything simple. It is always over engineered. Which generally isn’t a bad thing.

I’m with RMike89 on this one. I’m all for experimenting with things but potentially damaging a 416 upper would be painful. Are you looking lighten it to save some weight or just totally wanting to ditch the piston? Geissele makes a rail that should lighten it up a bit if that’s your intent.

[jpmuscle]

OP

Short answer is yes it’s doable. The conversions I’ve seen (owned one too) utilized a converted colt Scar H bcg. I do not recall if it was using just a custom gas key or not though.

This will allow you to use a standard BCG with the HK Piston system.

I’m not aware of a commercially available conversion that lets you run a standard AR barrel, gas block, 416 upper etc. you’re still stuck having to run the 416 style rail too

ETA now that I think about it Marvin Pitts May have done it but I recall his conversion was limited to converting standard AR barrels to work with the HK piston components


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[czgunner]

Why not trade uppers with somebody wanting a 416 upper.

[El Vaquero]

Why not trade uppers with somebody wanting a 416 upper.

He does not live in the USA. Sounds like wherever he lives there’s a lot of red tape.

[mack7.62]

The problem I see is the DI gas tube is smaller than the HK piston, seems like you will have to sleeve the hole in the receiver somehow and might me some alignment issues. One other thing to check is does the 416 use a standard barrel extension and nut?

[scdrnr]

Thanks for quick answers, all comments and aspects are appreciated!
I have to do what I can with what I can get hold of... As someone mentioned - a lot of red tape here.
Sometime ago I managed to source a 14.5" barrel, but haven't picked it up yet. I'm not sure which style it is but most proabably M4.
The inner diameter where the barrel extension slides into the upper is 1" - from what I've read thats the same as on the AR, which helps a lot.
Today I've also found a barrel nut with the correct metric treads + delta ring. :)
I'm not too worried about the rail sitting slightly higher and thus I'm not being able to have a continuous rail from the upper receiver all the way to the muzzle. My scope fits good on the upper and I'm most fond of the basic M4 hand grips anyway so that's not an issue for me as long as it doesn't create any other problems?

From what it seems: I have to make a new bushing for the piston hole with the correct diameter and/or cross shape to make a standard gas tube fit instead. Can someone please measure the gas tube hole with a vernier caliper or likewise?
The next problem is probably the length of the treaded flange where the barrel goes in. It's definately longer on the 416, but since all other variables seem to match I could probably just shorten it?

[call_me_ski]

60638

60639

[Steve Shannon]

Here are drawings with the dimensions:
https://www.scribd.com/doc/133024134/Blueprint-Upper-Receiver-M16A4


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[scdrnr]

Here are drawings with the dimensions:
https://www.scribd.com/doc/133024134/Blueprint-Upper-Receiver-M16A4


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Thanks! It seems doable! I guess I have to modify the fitting on the gas tube instead of the hole in the upper but it think it should work out fine.

[j-e91]

Any idea where i can find a barrel nut with the correct metric treads + delta ring ?