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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.
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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.