Black Mesa - How To Get The Shotgun And VERY Easily Unlock The Calculated Trajectory Achievement!
0In order to unlock the Calculated Trajectory achievement, you need to shoot kill 10 Headcrabs with the Shotgun while they are jumping. You can EASILY do this in Chapter 4 right after grabbing the Shotgun by exploiting a manual save.
Once you are in Chapter 4, go through the hole in the wall by the electric floor to find the lever that cuts the power. After the power is cut, walk across the water safely now and head to the locked door. Break the window to the right of the locked door and jump through.
Follow the hallway to the left, killing enemies as you go. At the end of the path, you will find a red door. Open it, go through, and then go straight across the bridge to where the Guard is. The Guard will get attacked by a Headcrab Zombie, so MAKE SURE to kill it before it kills the Guard.
Once the Guard is safe, he will open the gate for you. Follow the path to the back where the Shotgun can be found. After grabbing the Shotgun, make a manual save. A Headcrab will appear on the other side of the gate where you grabbed the Shotgun. Lure it into jumping at you, and when it does, shoot and kill it while it is airborne. Once it is dead, pause the game and load your manual save. Again, kill the Headcrab mid-jump and then reload your save. Repeat this process until you have done it 10 times and the achievement will unlock!
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