Unfortunatelly you can't get the rifle yourself, however in episodes 1 and 2 alyx is seen using it. In the mod "Missing Information" you can get the rifle, as the mod is supposed to immitate half life 2 beta, but you cant do the normal hl2 storyline. If you have Garrysmod, you can use the rifle by installing an addon, but the chapters will take a long time to complete due to the far extended loading times. In Smod you can also get it, and this is by far the best choice as it is basically the normal game but with extra weapons! Simply google your way to success for that mod and youll be shooting combine from afar with great ease in no time!