M4 Burst Cam Installation

The M4 Carbine is a selective fire battle rifle, which means it can fire single shots or three-round bursts, depending on the setting the shooter chooses for the fire selector switch. The burst cam is essentially a timing gear mounted on the side of the hammer that works in conjunction with the trigger group's burst fire disconnector. The M4 trigger group has two disconnectors, or small metal plates attached to the trigger: one for burst fire, the other for semi-automatic fire. Installing a new burst cam into an M4 requires disassembling the weapon and removing elements of the trigger group.

Things You'll Need

  • Gunsmith's drift punch
  • 2 wooden blocks, 2-by-2-by-4 inches
  • 1/4-inch wooden dowel, 4 inches long
  • Wooden or rubber mallet
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Instructions

    • 1

      Place the M4 carbine in a vertical position, with the back of the butt stock resting on a stable work surface. Push down on the slip ring at the base of the hand guards, then remove the hand guards.

    • 2

      Use a drift punch to press in the take-down pin as far as it will go. The take-down pin is located on the left rear of the receiver, just above the fire-selector switch. Swing the back of the upper receiver away from the lower receiver.

    • 3

      Push in the receiver pivot pin and slide it out from left to right. Separate the upper receiver assembly from the lower receiver. Set the upper receiver aside.

    • 4

      Move the fire-selector switch so it points to a position halfway between the "Safe" and "Semi" settings. Lay the lower receiver on top of the wooden blocks with its left side facing down. Position the blocks so they support the receiver on both ends.

    • 5

      Place the wooden dowel against the fire selector lever's mounting pivot on the right side of the receiver. Tap the dowel lightly with a mallet to push the fire-selector lever down and out of the receiver's left side.

    • 6

      Locate the hammer and trigger pin on the right side of the receiver, just above the trigger. Use a drift pin and mallet to push the hammer and trigger pin out of the receiver from right to left.

    • 7

      Reach into the lower receiver and pull the hammer assembly up and out. Gently pull the hammer spring away from both sides of the hammer, and then pull the old burst cam free from the hammer's left side. If the hammer clutch spring remains mounted to the burst cam, push it out with your finger. Do not lose the clutch spring.

    • 8

      Press the clutch spring back onto the left side of the hammer. Push the new burst cam into position over top of the clutch spring. Slide the hammer spring back into position on the hammer to hold the assembly in place.

    • 9

      Insert the hammer assembly into the lower receiver and align its mounting holes with the holes in the trigger housing and disconnectors. The disconnectors are two thin metal plates that project from the top of the trigger. Push the trigger and hammer pin back through the receiver from left to right to secure the assembly in place.

    • 10

      Reassemble the rifle by performing Steps 1 through 5 in reverse sequence.