I bought a new Daniel Defense MK18 mod 1 upper to run in a RR Colt M16A1 when it gets out of NFA jail.  I took my factory 10.5 Swithcblock Noveske SBR and put this upper on it. The Noveske has had several thousand rounds through it without a single issue. I took the DD MK18 upper removed the birdcage and replaced it with a AAC blackout flash hider to run my AAC M4-2000. The first shot out of this DD upper was a slam fire, or a three round burst to be more accurate. I immediately checked the FCG which is a Giessiele SSA-E. Hammer locks back all is good. Re assemble the next shot another slam fire or 3 round burst. I took the upper off and replaced the Noveske 10.5 CQB upper back in and it ran perfectly. I have not shot the DD upper again. I will contact DD on Monday but for the life of me I don't see how the upper could cause this. I did also check the firing pin and it was loose as it should be.  Any speculation as to how the upper can cause this?  Possibly the BCG is sitting too high to fully depress the hammer and engage the sear?  I don't see how this could be possible though.  

I'll take a stab. Switch out the bolts and see if it continues. Then switch the firing pins. Maybe a bolt or firing pin is out of spec and is sticking causing a slam fire.  As you know the AR platform has a free float firing pin and will ever so slightly touch the primer on chambering a round. Good luck and let us know!

Check the firing pin and firing pin channel. I had something similar happen recently. Firing pin was too long and there was a small piece of metal stuck in the fp channel.
Replaced the firing pin and all was good...

Mick

Put the BCG from a different rifle that works into your DD upper. If that works then take just the bolt out of the BCG and try that. Once you find the problem contact DD and get it fixed, do not keep the faulty part as people have got in trouble for possession of similar parts/guns.

Headed out to do just this now. Believe me I don't want to shoot it in the current condition any more than needed. I like being a gun owner and don't need that taken away for intentionally breaking the law. Hopefully I get it pinned down today.

Got to the bottom of the slam fire issue.  Technically not a slam fire I guess, the uppers was over gassed to the point that it caused the seat not to engage.  Before switching bolts I put in a red springco spring under the same H2 buffer. Unsuppressed the recoil was mild and smooth. I added the M4-2000 and the FA problem did not happen again I shot 80 rounds, no problems.  That said the upper is still severely over gassed. The recoil is violent for a 556.  Going to call DD Monday and see what their experience is with running this upper correctly suppressed.

I built my own mk18 upper but don't have any cycling issues.  10.3" DD barrel, Troy low profile gas block, BCM BCG, BCM upper receiver, DPMS stainless steel carbine gas tube, DD ris rail.

If it seems very over gassed maybe the gas port was drilled to large.

But an easy was to solve the issue would be to get a low profile adjustable gas block. There are a couple versions out there, usually adjustable by a screw driver.

I've been running a DD MK18 for about a month now (about 1500rnds so far) and I haven't had any issues. Although, I did switch out the buffer system for a VLTOR A5 system.

Is it possible you bumped the gun?

If you read all the posts above you will see where I say it is not technically a slamfire... It was a good way to word it, on a public forum, where eyes are watching. It sounds much better than "my semi auto has a three round burst setting its called pulling the trigger". The extreme overgassing was causing such a violentfast cycling of the action that the sear was unable to function properly. I slowed it down with a red spring and the three round bursts stopped.  

 

What supressor?

 

762-SDN-6

Haven't had any issues running with it either

Thanks for the info.