It has to be 1000 War Games only to unlock the Raider DSST helm. This does not include Spartan Ops, campaign, or custom games. I found this out the hard way, because in-game it said I had 1001 War Games matches complete, but it was including all of these things in the total. The helm still didnt unlock. In Waypoint it says I've only completed 800 War Games.
"The most important thing is to never stop questioning." -Albert Einstein. Xbox Live: AlteraLateralus