: A "long" or enhanced firing pin is critical to reliably ignite hard primers commonly found in surplus or steel-cased ammunition.
: Use caliber-specific magazines like those from Duramag (C-Products Defense) or ASC. Standard AR-15 magazines will not feed 7.62x39 reliably due to the tapered case body.
: If you experience feeding jams, some builders recommend using a Dremel to blend the standard M4 feed ramps into a single, wider "Bushmaster style" ramp.
To put together a useful piece based on a 7.62x39 configuration (often referred to as an "AR-47"), focus on to address common feeding and ignition issues inherent to this caliber in the AR-15 platform. Essential Build Components