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
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