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From HalfLife 2 Knowledge Base

The AR2.

This is a XM-29 SABR OICW (Objective Individual Combat Weapon). I have no idea why it was deleted, and would have made a fine addition to the game.

AR2 = Pulse Rifle?

In Valve's Hammer World Editor, the Overwatch Standard Issue Pulse Rifle (OSIPR) is referred to as an AR2