I have this slow mo. Its not the 7th rounds but its might give you some clue.Timing for the enbloc release/bolt closure. It is a go/no go gauge thats inserted into.the rifle like a enbloc is. But if your enbloc isn't prematurely unlocking might be a dead end aswell. (Hard to see exactly in your video)
You mention that a certain type of ammo is cycling fine without this issue, im curious if that ammo is "hotter" and giving the rifle more gas to cycle with. And this 7th round failure to feed is fron a short stroke where the bolt doesnt fully clear the round and is causing that tip up and dent.
Any measurements on your op rod Piston tip? Gas cylinder inside diameter is harder to measure unfortunately.





















































