Looking at the 50 BMG uppers I have seen that fit on ARs, it appears that you could potentially fire these with the bolt forward but in the unlocked position.  Is this the case?  Are they all like this?

My lower is in pieces currently, but looking at the upper, forward, but without the lugs engaged, I would think that it would not fire.if it did actualy rotate the bolt closed enough to engage the firing pin, I wonder if the hammer would have enough force left to ignite the primer?  

I suppose if it did get to the firing pin with enough force, the lugs would be near fully engaged....

ANdy

Which make of upper is that?

So yours looks like it cams the hammer when you lift the bolt handle, like the Serbu rifle.   On the ones that I seen, the hammer was cocked when the bolt was pulled back.  Not sure which make though.

UL50 by ligamec, and yes, opening the bolt cocks the hammer.