Here is a build I did with both a Black Rain upper and lower and Black Rain 3R 20" spiral fluted barrel. Generally, I use NiB BCGs; this one has a Fulton Armory BCG (same as a Colt).
In the picture at the bottom is a Fail Zero NiB BCG and a RGuns brand BCG? Can you tell the difference?
Hk USP 45 Expert, USP 40, USP 9, SIG P226, S&W M&P9, Hk 4, Makarov, Desert Eagle 40, Beretta FS92 Centurion, Kimber TLEII 1911A1, Glock 22, SIG P225, 1943 Rem Rand 1911A1, S&W mod 64, Walther PPQ, SIG P229, Browning BDA 45 (SIG P220), Hk45, SIG P230