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HK MR556 upper on Colt lower help--action binding?

2K views 7 replies 6 participants last post by  Kahuna 
#1 ·
I recently got a custom (14.5" SOCOM profile barrel) MR556 upper receiver in a trade. After looking for a MR556 lower for a while and having no luck (at least no luck at a reasonable price), I decided to get an "M4 Carbine" marked Colt Defense lower because I figured if I couldn't make a decent clone of what our Tier 1 .mil units are using today, I could at least make a decent clone of what they were using in the early/mid 2000s (when operators were originally putting their HK416 uppers on their old M4 lowers).

So I did my homework on HK556 upper/Colt lower compatibility and read about the hammer issue, but nothing else, and I sourced an un-notched milspec hammer. I finally put it all together over the weekend, but when I went to charge the carbine after assembly it took quite a bit of muscle to cycle the action--at least twice as much force as is required on my other carbines. And if I apply the charging force with any angle or cant (meaning I'm not applying the force 100% straight backwards), the action sometimes won't cycle at all.

Now I appreciate that the HK MR556 buffer tube and spring is beefier than a standard carbine setup, but this is pretty extreme (so no, "more gym time" isn't the answer!).

What am I missing? Some questions/possible theories I have are:
1. the charging handle. Sometimes it feels almost as if the charging handle itself is binding/getting stuck as I pull it to the rear. Should I be using a standard milspec charging handle and not the beefier HK-supplied one with the Colt lower?
2. the stock. is the fact that I'm using an old-school CAR stock on an older 4 position tube (as was commonly seen on the M4 lowers than the early 416 uppers were on) a factor?
3. the buffer tube. Would it matter if the buffer tube were commercial diameter and not milspec diameter (it's an old one and I don't remember which it is!)?
4. Other ideas?

Thanks in advance for the help!

Kahuna
 
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#3 ·
Uuuummmmmmmmm, you got me stumped. My 416 upper is on a Colt lower, with a machine gun hammer (non-notched/mil-spec), and I have a mil-spec tube/receiver extension, but that is mostly for my 416 stock.

without some pictures, I have no suggestions.... it sounds like something is binding somewhere, but I can't think of where that might be.

On a side note, make sure you're running an HK buffer and spring, sounds like perhaps you are, but the orig post doesn't specifically say that....

good luck
 
#4 · (Edited)
Yes, it's kind of bizarre (and i am running the correct HK buffer & spring).

I switched the charging handle with one of my DI guns and while it's still not easy to charge with my off hand, it's definitely better. I'm going to lube the bolt up some more as well-maybe that's part of it?

ETA: I will also try a different stock/buffer tube: I think that the buffer tube on it now is most likely commercial spec, which if slightly smaller, could definitely be a factor or since the HK buffer spring coils are slightly thicker than the standard…
 
#6 ·
Try lubing the hell out of the charging handle and cycle it a few times. When I first got my MR556 it bound up when I racked it the first time due to the charging handle. The CH had streaks/wear marks down each side. After some cycling, it wore in and has been trouble free since.
 
#7 ·
at least twice as much force as is required on my other carbines
I'm used to Colts and that's about right. I have a MR556 upper on MR556 lower and get the same thing. It runs like a top so I don't
think anything is wrong. My CH seems like it binds but slides free in the upper as does the bolt carrier. buffer slides easy in the tube. Of course I can't compare without actually charging your rifle.
 
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