Just got one to try out. Absolutely thermo fit with three different barrels I tried. Cheaper than BCM and in stock fairly often. Thought the Hide might like to know.
bkingsfirearms.com

BKF M4 MOD-0 Assembled Upper Receiver – Black (Light T-Marks) (Blem)
The BKF M4 MOD-0 Stripped Upper Receiver is a step up from our regular BKF-15 stripped uppers. The M4 MOD-0 uppers are held to tighter tolerances, Dry film lubed and tight fitting when paired with our M4 MOD-0 Lowers. Forged…