Does anybody know how much of the HK MR556 is compatible with mil spec AR-15s such as Colts and BCMs? I know that many aspects of the design were changed to meet the 1989 import ban standards, but I would like to know what parts can be changed out with commonly found US made parts. Mostly I’m talking about stocks, grips, lowers, charging handles, pins, …etc. Also, is there a spec sheet on the differences between the 416 and the MR556? Thanks.
Stocks (Mil spec), grips, flash suppressors, lowers, push pins, charging handles, dust covers, mag catch/release can all be switched out with regular AR equivalents.
Guys in Europe have replaced the firing control group as well, but depending on the unit, you may lose the ability to put the MR556 on safe when it is not cocked. I’ll let others with first hand experience on triggers chime in on that.
The only truly proprietary parts required to run the MR556 that I can think of are the BCG (the MR556 BCG is different and not compatible with an HK416 BCG), railed forearm, and recoil spring and buffer. The recoil spring and buffer are heavier than their respective AR components and designed for reliable operation, but Youtube has shown people firing HK piston uppers on lowers with standard AR recoil spring/buffers to no ill effect. Not sure what the reliability in the long run would be though, and since the HK recoil spring is supposed to be rated for 25K rounds, why bother using anything else.
As Hunter Rose described.
Regarding triggers, any AR15 will disable HK “safe on un-cocked” ability. Also to work with HK firing pin safety (same as in HK416) full heigh hammer (like in select fire triggers) is requites. Geisselle will not work. JP trigger works, same for CMMG 2stage. Several other (like Wilson or AR Gold) looks like will work, but I not tested it.
Pins, springs (except of firing pin spring, not present in AR) are same. HK locking (anti-play) take down pins can be changed for regular AR15 ones.
Recoil spring is of increased power, but regular M4 spring works. For weak, “hunting” ammo putting regular spring may be necessary, as HK buffer and spring are designed for 5.56 NATO round and possible use of suppressor.
Buffer is (very) heavy type using HDTP filling. Regular buffer works, also filling can be moved (it is powder) to other buffer body, for eg. rifle buffer is rifle stock is used.
Stocks, grips, etc. are fully exchangeable with AR15. Original HK buffer tube is of “commercial” size, but not mistake it with cheap tubes. It is increased strength tube, same as on OTB versions of HK416. HK went into bigger size, to increase strength.
Charging handles are interchangeable, however HK one is very clever and strong (it was designed for M27 IAR). BCM/Vltor works.
“HK only” are: upper receiver (AR15 lower can be used), barrel nut, railed handguard, complete BCG, barrel extension (with supported extractor), gas block, piston rod group, piston with rings (same as in G36/SL8).
Differences to HK416: lack on “fun switch”, barrel extension and BCG (both modified so HK416 BCG will not drop inside MR556), ultra-heavy profile match barrel (heavier than in M27), no chrome lining, locking take down pins, hanguard screw for hex wrench, shaved bayonet lug and sling loops from gas block, hex wrench screw that allow to lock down stock in desired position.
Hopefully I did not forgot anything :suicide2:
Thanks for all the details, guys, I really appreciate it. My agency has recently authorized the MR556 for duty use and I am weighing the positives and negtatives of using the HK. In regard to the barrel, I understand that it is made of a premium steel which is hammer forged and uses polygonal rifling. I’m assuming this process makes for a longer lasting barrel than that of a typical hammer forged, chrome lined barrel with standard rifling?
Geisselle triggers will work in the HK416. You need to specify your requirements when you purchase from them.
Member of m4c asked recently about that, and response from Geisselle was that they do not make and do not have plans to make triggers for MR556A1 or HK416 (except SSF, that works in HK416 by default as any select fire trigger will).
Barrel in MR556A1 is cold hammer forged with land and grooves rifling, from premium, rust resistant alloy steel specially formulated to use in firearms. Barrel is forged at HK plant in Oberndorf with chamber, then it is finished and profiled in USA by Daniel Defense. This makes barrel “made in USA” for legal reasons. HK makes polygonal barrel in 5.56 for G36/SL8, but I heard, that now they established that for small calibres land and grooves work better, even if deliver lower velocity than polygonal rifling. All 416 family rifles use lands and grooves (rifling is made as part of forging, it is not cut in separate step).
Cold hammer forging can be, as everything, made good or bad. Also steel used can be good or bad. HK is known to make excellent CHF barrels from very good steel. They, however, do not have monopoly for great, long living and accurate CHF barrels. For example SAKO makes great ones too. I’m sure there are also good ones made in US, I just have experience with HK, SAKO and FB Radom (Tantal/Beryl) CHF barrels.
“any select fire trigger” will not work in the hk416…
It is apparent www.experts, w/o familiarity with the topics at hand, will continue to pontificate about which they know nothing. I will leave the threads to the ill-informed bottom feeders
Tell me more. I’m open to learn new knowledge every day. Any particular trigger on your mind?
What I seen is that HK416 upper works in full auto when combined with M4A3 lower or SR16 lower. Those seem to have mil-spec select fire triggers. I assumed that select fire triggers for M16/M4 family are made per some standards. I might made wrong assumption.
Obviously select fire trigger will not work with MR556A1 (at least not in auto or burst - carrier will not work with auto sear).
Please keep posting. It is the absence of knowledgable posters that a forum declines the way other forums have.
Just buy an MR556 upper for $1600-ish and plug it on a good quality AR lower so you can use any standard AR parts, pmags, etc. The lower is more trouble than it’s worth.
AFAIK nothing on the upper exchanges with the AR except the charging handle, flash hider and any rail accessories. This is the primary reason why the 416 actually works and keeps on trucking compared to most other “piston conversions”, it’s not a “conversion” and that’s a good thing.
I’m not at liberty to post someone else correspondence. But not long ago I pointed someone to this information. This person asked about SD-3G trigger for HK MR556A1 and got answer that there are no plans for any SD trigger for MR556A1. So this “full line of triggers” are misleading as well. From what I know, SSF trigger works with 416 and SSA trigger could be ordered with modified SSF hammer to work in 416. I was pursuing this matter for my MR some 3 years ago. I settled with JP 3.5T for time being.
BTW Still waiting for info on M4/M16 select fire trigger groups that do not work with HK416 upper. Some people I know could value this information.
EDIT: armakraut: dust cover exchanges 