Yes, the Mk262 was designed for the SPR (Mk12). Yes, the Mk262 performs great in many rifles, including 10.3" Mk18.
I am not privy to their steel, but I know enough about barrel construction to say that the actual steel is not the same barrel maker to barrel maker. I have seen the barrel damage, one barrel blank manufacturer to another, inside the bore at the gas port, and the Douglas barrel has just worn less and lasted longer. This has been documented. I assume it is the steel. I am not sure what else it could be.

