This is an Easter egg found in the Halo 3 Beta and in Halo: Reach.
When a non-scoped weapon is zoomed-in, a line of binary code is visible in the lower-left corner. In both games the numbers read 01000100011000010111011001100101. When translated from ASCII eight-bit binary code, the code reads "Dave", a reference to Bungie interface designer Dave Candland.
This achievement may be glitched for some people. If that happens, try Vrruumm's suggestion: "I set my console to offline. Deleted my local save. Loaded up the game, then redid the ADS in a custom game. Turned my console back online. Connected to XBL. Closed out the game. Loaded the game back up. Critical! Loaded the older save. Achievement popped while syncing save. You might just have to delete the local save and resync it, but the way I explained is the way I got it to pop."