No set company, I've used all sorts of brands. I think much of it comes from only a few places, but made to different specs.
Unfortunately I suppose, you do get what you pay for, and in my experience the higher priced stuff IS better. Better tolerances anyway, fit and finish. The best upper and best lower I've ever owned were by far the most expensive,
Personally... I DO NOT like Olympic arms or DPMS. I say that not as a rifle snob, but because I own and have owned products from both companies. I stand by that statement, with the ironic part being the upper I just bought the other day is an Oly arms, and I often buy DPMS parts because I can get them cheaper. But for what they are, mostly small parts, it really doesn't matter. Don't get me wrong, both companies rifles will work and shoot fine. They're just not as nicely put together or finished as the others. I'd take DMPS over Oly.
There are companies to avoid... avoid anything from Blackthorn/Vulcan/Hesse. (run)
The upper/parts I had from Model 1 Sales seemed very cheaply made and flimsy. i wouldn't recommend them either.
Of the big names... I haven't seen much current Colt stuff so I can't really comment on it. Based on some of the
older ones they wouldn't be my first choice. (and you do pay for the name a little) I would take Rock River Arms over Bushmaster, but thats based on minor differences in finish. S&W, Armalite, Stag... you can't go wrong with any of them.
The smaller shop stuff is a whole other ball game and there's a million companies selling parts. But price is a good guide. It's
ALL over priced, but there is a noticeable difference in a lot of it. I've bought the cheapest lower and the most expensive lower and there is a big difference... but they all do they job about the same. It comes down to fit and finish.
FWIW.. i think the Stoner design is a HORRIBLE choice for a combat weapon.
