Curious if anyone has both an HK416/MR556 bolt carrier and an Adam's Arms piston bolt carrier and can compare the two? If so, is there any chance you could check if the dimensions are the same from the front of the bolt carrier to the "gas key" protrusion where the piston strikes the bolt carrier when cycling the action?

From pictures I found online, the front of the two carriers look extremely similar.

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Any and all help is much appreciated!

one significant difference, you can see in your pics, the HK tombstone has a shallow dimple and the op rod has a corresponding rounded end.  The AA tombstone is perfectly flat, and the AA op rod is correspondingly perfectly flat.

HK/416 on left. AA does fits 416 receiver but does not close. Seems the bolt recesses slightly further into the carrier on the HK/416.

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Thank you both! I have a brownells hk-compatible bcg on order, but was really hoping the dimensions would line up so I could go ahead and get my other parts together. Patience will have to hold out.