If you buy your Stag AR from a dealer that knows what they are doing, they should have already staked the gas key for free.
There is no such thing as 5140. It is 4150 barrel steel. There are several grades of this. The military uses three different types (4150 ORD, CMV and 4150 Resulfurized).
This barrel steel really only matters if your running the weapon on FA as it handles heat better.
C4