Why do you need an FFL to buy a BRP mg-34 upper that fits on ar-15 lower. I thought that the lower was considered the firearm.

In the case of the xmg99 upper, ATF has determined that it's capable of firing a shot with no lower attached, so it's considered a title 1 firearm all by itself.

So is there still hope of haveing an ar-15 beltfed upper that is not considered a firearm all by itself? What about the 50 bmgs that are being made to fit on an ar-lowers? are they capable of being fired without a lower?

I can only assume that if the ATF determines them to be capable of firing a shot while not attached to the lower, they'd be classified as firearms, also.

I'm working on a beltfeed adapter which would definitely not be classified as a firearm by itself - if I can get it into production, that would fill your bill.

I don't think the Ares shrike has been type-classified yet, since it's not been offered for general sale, nor been put into production yet, to my knowledge.  If Ares can ever get that into production, it might also fill your bill.

Does the XMG99 fire from an open bolt?

I have an MG-34 parts kit.  If you were really stupid you could fire a round (and probably blow your hand off in the process) with just the bolt and barrel.