"This anti-life form weapon accelerates and fires highly charged particles."
The Charged Particle Rifle is a weapon that can be found in the underwater Missile Silo. It has been created to eliminate any B.O.W. and is required for the fight against Morpheus. It has to be charged, and then can be used infinitely and can take out almost any B.O.W. in a single shot. After the first form of Morpheus the rifle loses charge and cannot be reused.
This weapon can be obtained after defeating the game in Hard mode.
Even if a player has it from the beginning, It wont do anything to Morpheus, just like a standard fire arm.