To agree with the rest, it doesn't matter who makes RRA's lowers, since you're buying their product, if it's out of spec in anyway, it's on RRA.
That said, DPMS has great lowers, so I don't even see that issue.
For the record, I think Oly is the only one that makes everything in house, while I think Oly does fine, I'd take A,B,C,D or RRA over them any day.
IIRC, Bushmaster has their whole receiver subbed (some to LAR), Armalite and Colt machine their own for forgings. Not sure on DPMS.