It is the barrel itself. Barrels offered in 4150 Chrome Moly Vanadium and 416R Stainless Steel.
We build our 308 M5 platform to the AR308 pattern, with a DPMS High Profile (.210) Tang rail and 1-7/16 x 16 TPI. Please note that our .308 products are machined to AR .308 specs, which is similar but not the same as DPMS Gen 1. Unfortunately, because there is no set standard, we are unable to guarantee compatibility with any other manufacturer’s products within this platform
Aero Precision's M5 upper follows the DPMS High pattern.
Aero Precision's M5 upper follows the DPMS Gen 1 High pattern.