I'm going to be building something specific on a GBBR platform in the near future, and I haven't settled on the brand yet because the deciding factor is what exists for straight profile (no M203 cuts or C8 barrels) 16" barrels that allow me to keep the standard length M4 handguards (7", not free float). Basically, the barrel needs to be able to drop straight into a standard M4A1 while keeping the rest of the front end.
I found
this one, but otherwise there's a lot of stuff that I'm not entirely sure about.