My experiences so far.
With 4 older Bushmasters fit and function are flawless except the lowers w/ serial #L0691xx, L07372x and L07375x, there is no beveling on magwell. This does not matter if you are using USGI mags, problems will occur with Thermold mags (which I don´t touch anyway).
Newer L3035xx lower has the beveling. All of them are DIAS compatible.
RRA upper&lower set bought from Brownells couple of years ago had annoyingly tight fit between upper and lower. Magwell was also too tight, I had difficulties Center Ind. magazines. Sold it later. Not DIAS compatible.
Though I consider myself as Bushmaster guy my latest purchase is
DPMS forged lower from Brownells. I´m pretty pleased what I see. Fit and finish is good with Cerroforge uppers, no machining marks on exterior and yes, it is DIAS compatible.
MN