I'm told that Bushmaster is more of an assembler of subcontracted parts than an actual manufacturer of those parts. Accurate or no?

As are most other manufacturers.

pretty much all AR rifles are "parts guns."  thats just the way it is.  

i think theres maybe 3 or 4 companies total that make the parts (upper/lower recievers) for about 20 different companies.  if you feel like taking the time, go to a gun store with a large supply of AR rifles and start looking on the port side of the upper, just under the rail/carry handle.  bushmaster, dpms, olympic, colt, rra, doubestar, cmt, cmmg, and a few others usually have a keyhole emblem.  all those different companies' uppers are mad at the same place.

its not a bad thing. . . part interchangability is great, and it keeps everyone mostly on an even playing field.

That said, Bushmaster does produce their own parts, just not all of them.  Their barrels and many of the upper and lower assemblies are made by Bushmaster and they make a very high quality product, that's why Magpul is having them produce the Masada.

Bushmaster makes their own barrels,uppers im not sure if they machine their own from the raw forgings.I know BFI marked lowers with the 3D snake are machined in house,1 sided snake with L prefix ser# are LAR machined lowers.