I am thinking about taking the plunge into .50 cal.  I have some spare lowers hanging around, and it seems like a .50 upper is the cheapest way to go.  However, in my mind the real reason for buying a .50 cal is very long range accuracy.  How do the .50 cal uppers which fit on an AR lower compare with a dedicated .50 cal like the Serbu or AR-50?  I would have no interest in a 50 BMG which could not consistently shoot at least 1 moa reguardless of the price.  Any knowledge/experience appreciated.  Thanks in advance.

the http://www.50bmg.net ALS upper is probably you best bet for now.
It's been winning at the FCSA matches recently.
The long barreled Ferret SuperComp has done OK too.

A stock BFG recently won a match for a 1st time competitor too.

Most of the other uppers are geared toward the lightweight short barreled market for hunting/plinking out in the boonies.  The 22" UltraMag I've got set up with a holosight is pretty quick and accurate out to 500yds.  But I've never really sat down to test it with a scope beyond that yet.  They (and the Ultralite50 or Anzio) are available with longer 29" barrels, but I've not yet read of anybody posting a review with such a kit.

Check out www.anzioironworks.com they have a complete gun for 1900, that's with fed exercise tax paid.  Mine shoots sub moa and  I love it  the best gun out there for the money.   Kevin