Don't overlook the Double Star ( J & T Distributors). I have two of them, they function and shoot as well (or better) than Armalite, Colt, or Bushmaster. I have a 24 inch heavy stainless barrel ( 1 in 8 twist) on one of the Double Stars that consisitely deliverys groups of less that a quarter at a hundred yards. LARGE you say, perhaps, but not bad for my 70 year old eyes.
Looking at the parts available, and the diffrent guns I am of the opinion that MOST of the major parts are made by just a handful of producers. The lower receiver may be milled by many diffrent shops, but barrels, internal parts. etc, are made by just a few.