I have an issue that's driving me crazy. If the pull back on the charging handle and let go or push the bolt release on an empty chamber, my geissele mk8 moves backwards. Shoot it and it will move back forward under recoil.

Theres a small gap between the upper and the rail that i can see with light behind it. Drop the bolt a few times and its closed up tight. Ill upload pics as soon as photobucket decides to work.

Originally i contacted Geissele and sent the upper in. They reinstalled it and sent it back. Same thing, sent it back to them. They checked everything and said it was well withing spec, reinstalled it and couldn't duplicate the issue. Got it back and the same thing happens. So I reinstalled it and if i drop the bolt or charge it on a round its fine or if i ride the charging handle in its fine and doesnt move.

Geissele sent me a new hand guard that i will be installing. But its on a 14.5" pinned barrel, so ill have to have the barrel nut swapped and a new flash hider pinned. So i was going to see if any body had an idea of something else to check.

I had the same exact issue with this gun and an eotech exps. Swapped to a t-2 in a adm mount and seems to be fine now but now im wandering if something else is out of spec.

Ive tried a few thing but that doesnt mean that i didnt get 2 out of spec parts.

The gun is a build with everything from bcm except the rail. Bcm lower, upper, bcg, charging handle, 14.5" bfh mid length barrel, gas block and has tube. Rail is a Geissele mk8. Flash hider is a silencerco asr flash hider pinned and welded.

Any help or ideas would be awesome

Is the new handguard Geissele sent you the same as the one that’s already on the gun?

If so, have you tried swapping the new rail onto the old barrel nut to see if the issue persists?