Boss Hoss,
In discussing this with my manufacture that will be mass producing my Raptor, the receiver body, rail base and recoil lug is 416R 34-36 RCh. 15-5PH is a bit easier to machine but even at 42-44 RCh it still has to high of a potential to gaul.
416 virtually eliminates the gauling and with new cutting technology and high pressure coolant through the spindle the raw bars can be heat treated before machining.
The complete one piece bolts are made from aircraft certified 4340CM. Again, this alloy with proper heat treating offers good corrosion resistance but also is very good for dramatically reducing any chance of bolt lug gauling under normal use conditions. Dramatically superior to a stainless bolt and receiver which always have gauling issues.