While I would say that "A,B,C, and R", will all put out a good product, I don't think I'd go so far as to say they are all exactly the same.
For example... Barrels. Colt and Bushy use 4150 steel, while others use 4140. (has to do with resistance to heat, but thats about all I know about it
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Bushy chrome lines all their "normal" barrels and chambers. AFAIK, no one else does this. Colt and RRA tend to put out one chrome lined model, and thats it. Chrome lining can be very important to reliability in adverse conditions.
Colt tends to put out non-standard lowers that have non standard fire control parts, non standard pin sizes, blocked autosear areas... really odd stuff. I imagine that this can be a pain when sourcing parts for a Colt lower.
Not everyone's carriers are the same. I think Bushy's semi-auto carrier is the closest semi-auto carrier out there to the M16 carrier. The underside rear portion of the carrier is only milled back a little bit, where Colts are milled all the way off. The firing pin is also fully enclosed rather than partially exposed, with Bushy vs Colt. Colt has also started using plastic buffers for some reason, instead of the standard metal ones.
At the end of the day, I think that you will find that Bushy's lineup is the closest to "mil-spec" that you will get off the rack. The only thing better off the rack is perhaps Colt's MT6400C offering, which is literally a civilian version of an M4 in every way.