The 8640 alloy has the right  yield strength (150 ksi),   sufficient impact toughness (65 ft-lbs) , and acceptable hardness ( 35 Rc). 

In addition the transition temperature is -140F, equal to the transition temperature of the original 4340 alloys.

An additional benefit is that the 8640 is a lower cost alloy than the original 4340, because the 8640 has a lower nickel content than the 4340.