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Bushmaster Gas Piston Retrofit Conversion Kit Installation Instructions


The Bushmaster Gas Piston Retrofit Conversion Kit is a patent-pending gas piston system for MIL-Spec M4 type carbines (Note: MIL-Spec carbines have a barrel diameter not exceeding .875" immediately behind the front sight block). It replaces the original gas impingement system tube and bolt carrier with a robust short-stroke piston and operating rod that is designed to enhance the controllability and reliability of your rifle under adverse conditions. The Gas Piston Retrofit Conversion Kit is designed for easy installation requiring only a few minutes of time. However, if you are not thoroughly familiar with the disassembly and function of this type of rifle, you should have this kit installed in your rifle by a trained armorer or competent gunsmith who is well versed in the AR-15/M16 family of weapons. Such a trained armorer can perform the test-fire procedure to ensure that this kit has been properly installed and is completely functional.

gas piston kit.PNG

gas piston installed.PNG                        handguards.PNG


Please Practice Safe Firearms Handling.

Improper handling or misuse of firearms may result in property damage, bodily injury or death. Before beginning the installation of this kit, ensure that your firearm is unloaded-remove the magazine and check that the firing chamber is empty.

Bushmaster Gas Piston Retrofit Components


gas piston components.PNG



View # Part Name View # Part Name
1 Solid Striker Bolt Carrier 8 Gas Piston Housing/Barrel Clamp
2 Operating Rod Receiver Bushing 9 Gas Piston Housing Note: the gas piston housing is comprised of the spigot (fits into gas block) and the gas cylinder. These are factory assembled as a unit, and should not be disassembled.
3 Flat Washer 10 M4 Handguard Set (not shown at actual size) Note: the lower handguard half has double heat-shield, and upper handguard half has the single heat-shield. This is required for clearance over gas piston housing and clamp.
4 Operating Rod Return Spring 11 O-ring
5 Operating Tappet Rod 12 5/64 x 5/16 Coil Spring Pin
6 Connecting Link 13 #8-32x.5 Socket Head Cap Screw
7 Gas Piston


The Spigot Spring Note: the spigot spring is inserted into the gas block. A new roll pin is included.

Installation Procedure

  1. Remove the ammunition magazine and all ammunition from the weapon. Visually inspect the chamber to ensure that no ammunition remains in the weapon.
  2. Remove the handguards, charging handle and bolt carrier group from the weapon observing normal field stripping practices. Remove the bolt, cam pin, firing pin and firing pin retaining pin from the bolt carrier body. Remove the 3 gas rings from the bolt-they are not required for function with a gas piston system. Set the original handguards, gas tube, bolt carrier and gas rings aside. They will not be used in the gas piston conversion kit assembly.
  3. Using a small drift punch and hammer, drive out the roll pin that secures the gas tube to the front sight/gas block and remove the gas tube from the upper receiver assembly.
  4. Using a suitable chamber or bore brush made of copper, clean the counter bore located inside the upper receiver housing (just above the barrel extension).
  5. Insert the tapered end of the receiver bushing into the counter-bore of the upper receiver and press straight until it stops (see Figure 2). The operating rod may be temporarily inserted as shown to help align the receiver bushing during this step.

gas piston fig 2.PNG

  1. Slide the operating rod return spring and flat washer onto the operating rod and insert that assembly through the front of the delta ring and into the upper receiver (as shown in Fig. 3).

gas piston fig 3.PNG

  1. Insert the spigot spring, and then the gas piston housing into the gas tube hole in the front sight/gas block (as shown in Fig. 4). The spigot spring should be compressed by pushing the spigot forward until the roll pin hole in the spigot will align with the hole of the front sight/gas block. (Note: The purpose of the spigot spring is to handle the impact loading of the operating rod during counter-recoil, instead of transmitting this loading to the roll pin). Once aligned, insert the new roll pin into the front sight/gas block and use a small drift punch and hammer to seat the roll pin.

gas piston fig 4.PNG

  1. Once the gas piston housing is pinned in place, assemble the pieces of the gas piston housing/barrel clamp around both. Insert the allen head screw and tighten.  (Note: Loctite can be used on the screw to prevent it from "shooting loose"). Make sure, when positioning the clamp, not to block the gas vent holes in the Gas Piston Housing (see Fig. 4).
  2. Compress the operating rod and spring rearward into the receiver and place the connecting link over the operating rod as shown in Figure 5.

gas piston fig 5.PNG

  1. Grasp the knurled surface of the connecting link and draw the assembly rearward a sufficient distance to permit the rear of the piston to be placed over the small end of the connecting link, as depicted in Figure 6. Once this is accomplished, align the front of the gas piston to guide it into the gas cylinder. Allow the entire assembly to move forward under its spring force until it stops. Apply two drops of Break Free CLP (or other suitable firearms lubricant) to the gas vents in the cylinder and check the gas piston assembly for free movement. The operation should be smooth with no binding or impedance other than that of the operating rod spring.

gas piston fig 6.PNG

  1. Reassemble the bolt, cam pin, firing pin and firing pin retaining pin into the high performance bolt carrier provided with your Bushmaster Gas Piston kit. Lubricate and install the charging handle and bolt carrier assembly into the upper receiver, and then close the receivers with the rear takedown pin.
  2. With the bolt carrier in its forward most position, pull back on the operating rod (using the knurled surface of the connecting link for traction) until it touches the bolt carrier. There must be a small gap (approximately the thickness of a dime) between the piston and connecting link. If gap is satisfactory, charge the weapon several times to check for normal function. Bolt carrier operation should be smooth with no abnormal resistance.
  3. Install the supplied handguards as shown in Figure 7.  The handguard half with the double liner must go on the bottom.  Assembly is complete.

gas piston fig 7.PNG

Test Firing Procedure

  1. Place the weapon on SAFE.
  2. Put a single cartridge into an empty magazine and with the muzzle pointed in a safe direction, insert the magazine into the weapon.
  3. Chamber the weapon with the charging handle, set the selector to FIRE and shoot the weapon. The brass should eject and the bolt should lock open automatically with the bolt catch.
  4. If the bolt locks open, then repeat the procedure using 3 rounds in the magazine.  If operation is normal, unit is functioning correctly and larger quantities of ammunition may be fired.


If the unit fails to operate correctly, unload the weapon and set to SAFE. If the unit fails to extract spent cases from the chamber, then the provided Viton O-ring should be installed over the extractor spring as shown on #11 in the components diagram. For other issues, review the procedures and ensure that installation was performed correctly and that the Bushmaster Gas Piston Kit installed matches the host weapon, i.e. barrel length, diameter, etc. If installation is correct and unit still fails, contact Bushmaster Customer Service for further assistance.


Periodic maintenance of your Bushmaster Gas Piston Retrofit Conversion Kit directly relates to the reliable function of your weapon. We recommend cleaning, and then apply a thin layer of oil to all moving parts of your weapon before any extended storage or every 500-1000 rounds. At this time the barrel clamp (shown in figure #4) should also be inspected for looseness, or movement over the exhaust ports.

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