I was interested to learn that HAO’s PTW 416 upper will go straight onto an OEM Systema lower, without fettling – and vice versa.
Some pics HAO provided, to prove the point:
You’ll also notice that the OEM Systema lower features a HAO 416 fire selector. Again, this is a drop-in fit.
It may strike some as odd that I’m mentioning this. Surely you can run any brand’s PTW upper with any brand’s PTW lower? Well, that’s a nice idea. But some manufacturers stray away from that vision, for their own reasons.
I think HAO’s way is very sensible. Use the original as the datum point, then weave your 416 look around it.