Standard chrome sockets will shatter under the violent hammering of an impact gun. They are made of chrome-vanadium steel, which is hard but brittle. Impact sockets use chrome-molybdenum steel instead. This alloy is softer and more ductile, allowing the socket walls to flex slightly under stress rather than crack. They also feature a dark, matte oxide finish rather than shiny chrome plating, which can peel off in sharp flakes during heavy use.
The socket walls are much thicker to absorb the heavy blows from the gun. The drive end has a retaining pin hole or groove to keep it locked onto the impact wrench anvil. If a socket fits loosely, it will waste energy and quickly round off the corners of the bolt. Never use a chrome socket with an impact gun. It is a serious safety hazard that can send sharp steel fragments flying across your shop.