Two of us have talked to Bushmaster, and have gotten the same response - send the problem uppers back. Their initial recommendation was to check the carrier key for leaks where it mounts to the carrier, but swapping bolt/carrier assemblies proved the problem to be not in the bolt or carrier. I've got a suspicion that the problem is in the front sight base - maybe misalignment with the gas port, or a restricted gas port from the loc-tite that is used to secure the site base.
I did notice when we were breaking-in 2 of them that the gas ports had very noticeable burrs that could be easily felt with the cleaning patches, but seemed to smooth out after 30-40 rounds. Is it possible that the burrs are being bent over the gas port causing a restriction?
One of our club members just got his back and will be trying it out this week. We have 2 more members wanting to order rifles to shoot on our teams, but are understandably hesitant to get the Bushmasters.