The encountered M1A1 Abrams tanks are always stationary, only rotating their turrets. They're typically found defending key areas that have been fortified by the HECU. Their primary attack method is explosive cannon fire, while some have attached Machine Guns.
In Half-Life, the Abrams is first encountered in the chapter Surface Tension. It provides fire support for a group of entrenched Marines. Another is stationed in the chapter "Forget About Freeman!", blocking the entrance to the Black Mesa Central Complex. It has a mounted machine gun on its front.
A second unmanned tank is found in the same chapter in a vehicle repair bay. This tank has a Machine Gun mounted on its top which can be used to kill the enemies. In order to progress. Gordon must climb into the tank's open hatch and activate the cannon to destroy the door blocking the path forward.
In Blue Shift, an Abrams is encountered in the chapter Captive Freight. It is placed on a train flatcar in the Freight Yard. Although it makes no use of it, this variant is armed with a TOW launcher, and is able to fully rotate its turret.
 Behind the scenes
The M1A1 Abrams was created by Steve Theodore and is based on its real world counterpart. The vehicle is built out out of brush-based entities. The source files for Half-Life' maps reveal some unused variants. The map
c3a1a includes two: one in place of the active tank seen in the final game but facing the opposite direction, and another with a different and more detailed turret. In the next map,
c3a1b, there was to be a second tank in place of the Energy Cannon.
 List of appearances
 See also
|Vehicles, vessels and transportation|
|Aperture Science transportation||Borealis · Teleportation|
|Black Mesa transportation||Black Mesa SUV · Black Mesa Transit System · Black Mesa truck · Rail cart · Teleportation|
|Black Ops transportation||AH-64 Apache · M44 cargo truck|
|HECU transportation||AH-64 Apache · Jet · M1A1 Abrams · M2A3 Bradley · M44 cargo truck · Osprey Heliplane|
|Combine transportations||Advisor Platform · Armored Personnel Carrier · Hunter-Chopper · Razor Train · Teleportation|
|Resistance transportation||Airboat · Mil Mi-8 · Muscle Car · Scout Car · Teleportation|
|Xen transportation||Alien Craft · Teleportation|
|Trainstations||City 17 Trainstation · Depot · Technical Trainstation|
|Other||Rail transport · Road transport|
|Black Ops (cut)||Osprey Heliplane|
|Combine (cut)||AirEx Truck · C-130 · Combine SWAT Truck · M2A3 Bradley · Merkava · Osprey Heliplane · School Bus|
|Resistance (cut)||Borealis · C-130 · Jalopy · Jet Ski · Osprey Heliplane · Submarine|
|Other (cut)||Digger · Stealth|
|Hazardous Environment Combat Unit|
|Characters||Minor members · Dwight T. Barnes · Sharp · Adrian Shephard|
|Classes||HECU Engineer · HECU Medic · HECU Sniper|
|Weapons||9mm Pistol · Artillery Cannon · Combat Knife · Desert Eagle · Dynamite · Fragmentation Grenade · Gun Emplacement · Land Mine · Laser Trip Mine · M249 Squad Automatic Weapon · M-40A1 Sniper Rifle · Mortar · Submachine Gun · Rocket Cannon · RPG · Satchel Charge · Sentry Gun · Shotgun · Tripod-Mounted Heavy Machine Gun|
|Transportation||AH-64 Apache · Jet · M1A1 Abrams · M2A3 Bradley · M44 cargo truck · Osprey Heliplane|
|Technology||Chargers · Powered Combat Vest · Radio · Tactical Map · Transportation · Weapons|
|Other||Hazardous Environment Combat Unit models · Military Police · Robot Grunt · Santego Military Base|
|Classes (cut)||Human Sergeant|
|Weapons (cut)||20mm Cannon|