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Discussion Starter #1
I just got back from the Range with my Son to test fire My New HK MR556 UPPER. Not so good. This is a Brand new Upper from HK PARTS.NET. I have two Colt uppers, a 6940 and a 6920 all colt parts. I am running Top of the line KAISER Defense lowers with all Mil-spec colt parts also top of the line. I mated the HK Upper with my lower and yes I switched the buffer and spring with the Supplied HK Buffer and spring. No fire. Feeds fine, Mags are good Extracts fine but when I looked at the round the Primer shows no sign of a Strike from the firing pin on the primer at all. I have a brand new HK Firing Pin and spring as a spare. Switched them out and the same thing no primer strike from firing pin no fire. Switched lowers I have two of them. Same thing no fire. Bolt looks like it is Locking up fine in Bolt recess lugs. Hit the foward asest several times. Still no fire. Switched mags also, still no fire. Put my colt uppers on the lowers and fired all day long no problems. I bought the HK Upper because I wanted a Top of the line Piston upper and because they are compatable with any good AR Lower. The hk Mates up fine with either lower and is nice tight fit. So what gives here? My Hammer springs are good. Do any of you here have any problems with your uppers on AR Lowers? DO I need a different FCG ? So what gives I need answers please
 

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I have read stories about non-HK hammers somehow interfering with some portion of the carrier. The so-called notched hammer theory, I guess. If there is no evidence the firing pin is hitting the primer, then the hammer is likely not striking the firing pin.

This was supposed to be an issue with 416 carriers, IIRC. I am no expert--someone who is will check in here this weekend.
 

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It sounds like your hammer not disengaging the firing pin safety. You need a full height un-notched hammer like a full auto hammer to disengage the firing pin safety on the MR556.
 

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It does not have to be a hammer from a full auto trigger kit, but it cannot have the notch cut-out commonly found on some manufacturers semi-auto trigger assembly's. Some semi-only triggers have the notch cut out, some do not.

My LMT lower parts kit had the notch cut hammer, thats why I had to swap it out. Its certainly not a problem, 416 uppers just hit a little higher on the hammer. It cost me nothing but $10 and about ten minutes to swap out.

Hammer style needed for HK416 uppers (Not necessarily a full-auto hammer - just without the notch cut-out on the face):


Hammer style found in my LMT lower - has the notch cutout on the face (sometimes incorrectly referred to as a "semi-auto hammer"):


If your semi-lower has the notch cut hammer U.S. Armory Gun Parts has the exact semi-only hammer you need for $15.
US Armory Gun Parts and Accessories

I've dealt with this guy many times in the past and he is good to go.
To help illustrate the above points
 

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*EDIT*

Nevermind. This post was stupid.
 

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It sounds like your hammer not disengaging the firing pin safety. You need a full height un-notched hammer like a full auto hammer to disengage the firing pin safety on the MR556.
This exactly. In the middle of your HK carrier just behing the anvil, you will see what looks like a hook that drops down behind the firing pin. This is the firing pin safety. When the hammer moves forward, the top of the hammer should push up on it to disengage the safety.
 

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Discussion Starter #7
Thats it my Hammer has the notch in it. Thanks a Million I am off to buy a new hammer. Thanks again
 

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So by doing this, you're saying you can run any trigger including the Giessele SSA-E?
Yes you could, but as I just found out, you open your gun up to light primer strikes every time you chamber a round exactly like what happens with a regular DI AR15.

If you don't mind the light primer strikes when chambering due to the weight of the firing pin, then by all means, do so and run any trigger you like.
 

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Discussion Starter #10
Replaced with new hammer with-out notch, went to Range all I can say is: FLAWLESS, this baby is way better than my DI Colt Upper.
 

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Replaced with new hammer with-out notch, went to Range all I can say is: FLAWLESS, this baby is way better than my DI Colt Upper.
Glad to hear :)
 

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Do not write it at M4carbine.net. They will skin you alive ;)
No they won't. Some are warming up to COLT's Gas Piston upper...

Strange, indeed.
 

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No they won't. Some are warming up to COLT's Gas Piston upper...

Strange, indeed.
That's because the Colt koolaid is particularly strong over there. There is an underlying current of "Pistons suck" over there, especially if it has HK stamped on the side, but the moment a piston gun comes out with the magical prancing pony on the side, all of a sudden this piston **** ain't so terribly bad. Some have even brought up that they *think* the IC competition is down to the Colt gun and the 416 and they hope that the Colt gun wins simply because it's Colt........even if the 416 is better.

Bring up the 416 and you'll still get lectured and lambasted.

Sigh.

Like i've said before, I like the place, but sometimes they make me want to stick my head through a brick wall.
 

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Like i've said before, I like the place, but sometimes they make me want to stick my head through a brick wall.
Even though they have some very heavy hitters in terms of knowledgeable end-users/experts, some of their honorary "experts" are highly suspect. Anything outside the BCM/Colt/DD/KAC standard and it becomes a slightly more sophisticated version of AR15.com (or "TOS" as they refer to it).
 

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Having a similar problem, but after dry firing I can't even charge the handle to pull the bolt back (frozen). It's stuck and I have to open it up, cock the hammer back with my finger, and try again. I get the impression that the hammer actually sits too high in this case, but I don't understand how this would prevent me from pulling the bolt back. The hammer is a Wilson.

This thing hasn't even been to the range yet. My buddy said to just fire it a few times, but the mechanics are baffling as to how this issue is going to sort itself out by firing it.
 

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The hammer is a Wilson.
What version? Only TTU-3G works with HK. Other have wrong profile of hammer and need machining before using in HK. Probably your hammer jams solid against firing pin safety lever and that is why you can move BCG back.
 

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Even though they have some very heavy hitters in terms of knowledgeable end-users/experts, some of their honorary "experts" are highly suspect. Anything outside the BCM/Colt/DD/KAC standard and it becomes a slightly more sophisticated version of AR15.com (or "TOS" as they refer to it).
That's why I joined. And then promptly purchased an Adams Arms piston upper for my AR 'cause DI isn't all that and a bag of chips.
 

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You are absolutely spot on. I'm replacing the Wilson with a Geissle SSA-E.

Strange, I called Wilson and they said that no issues been reported.
 
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