I've got a registered SBR lower and I'm considering getting a MR556 upper and having it chopped to 416 length (10.4"). Couple questions:

- Will it run with a standard AR lower? I've read it needs a standard AR hammer (no slot).
- I'd wanna run it suppressed - how do these uppers do suppressed?

I have (2) original 416 uppers, both 10 and 14.  

A standard MR556 upper will run on a mil spec AR lower.  Running it with a can should be fine.  I would send it to someone who has some experience with doing these conversions as there may be some gas port/block work that needs to be done.

Thanks - do you have any recommendations on who to send it to?

MR556 upper kit from HK, cut to 10.4",barrel reprofiled, 10" non-vented gas block installed. Lower uses 8.5oz Heavy Buffer and Wolff XP spring from here

Runs fine with or without the can. Locks open on empty either way. Recoil doesn't feel different from other ARs.

I send my stuff to Jayson Cotter at IGF.

Doesn't HK swag their rifle barrels?  Meaning that if you chop one, you're not matching the factory interior profile of the barrel?

Yes, HK barrels are swaged. How much, depends on overall length of the factory barrel.

Mine is similar, but different...

MR556 upper kit from HK, cut to 10.4", barrel reprofiled, 16" standard vented gas block installed. Lower uses 5.4oz H3 buffer and standard carbine spring... Runs fine with or without the can. Locks open on empty either way. Recoil doesn't feel different from other ARs.

I was supposed to get my MR556 to send to Jayson yesterday, but UPS lost  it and they can't find it. Last scan was 36 hours ago, and they opened an investigation to find it. I am going to be really upset if it lost/stolen because I got the last one from DSG until the new batch comes in whenever H&K feels like it!!

Marvin Pitts works on them too.

@NoScoE30 does HK barrels as well.  

Yes, the barrels are swaged, but it's pretty much a non-issue in practical use, plenty of folks with cut MR556 barrels with no issues whatsoever, and no complaints about accuracy.  

From what I can tell, people make a big deal about this and that combination of parts with MR556s... my MR556 upper was cut to 10.4", had the gas block plugged, uses a Surefire suppressor, and the HK factory buffer and spring.  No issues.  

Things like heavier buffers and vented gas blocks, while useful, were intended to improve life-cycle maintenance of guns having tens of thousands of rounds through them by CT units going through intensive training packages--while I'd love for my MR556 upper to be well tuned as I can get it, the reality is that I'm going to burn out my non-chrome lined cut down barrel long before I have the kinds of long-term wear issues that those things were designed to mitigate.  

As for "overgassing"... I wear glasses, not for protection, but out of necessity... and with the can, an ATPIAL, a light, etc. out front, along with the general "nose-heaviness" of the HK416/MR556, the carbine being "jumpy" or having too much recoil is really just not an issue I worry myself over.  

~Augee

I've only shot original HK416's and while not unpleasant, I much prefer the  recoil impulse on a DI AR.

Same... I mean, I know CAG, CIA, FBI HRT, DEVGRU, etc choose HK416s because they haven't found anything to perform better... perhaps the performance is needed when globetrotting from sea level to 14,000 feet in the Hindu Kush to the jungles of wherever...

but for a day time (sometimes night time) range clown like me, I like my govnah gas block on a standard 14.5.... made me picky with over gassed guns and sharp recoil impulses...

should you go with an MR556 conversion... Jason at IGF will sort you out better than anyone else I can think of

Yet CAG has a lot of DI Guns too...