AR Gas PISTON conversion kit for carbines


This is a complete short stroke GAS PISTON conversion kit for AR15 type carbines manufactured by Blackthorne Products. This system has a gas block that is machined from 4140 chrome moly steel (NOT cast). This gas block clamps onto a .750" diameter barrel at the standard carbine length and has a mil standard M1913 Picatinny rail machined into the top for easy attachment of accessories. The top of the rail is 1.200" above the centerline of the bore. The system features an ADJUSTABLE gas setting (I is for standard, II is for dirty gun, and III is cutoff) utilized by simply rotating the gas plug. The hard chrome plated gas piston is made from S7 tool steel for extreme toughness and long life. Also included is a bolt carrier with an integral gas piston lug. This lug is machined right into the carrier, it is all one piece of hardened 8620 steel. The carrier also incorporates an Anti-tilt design to ensure positive function. The handguards included with this system are M4 contour "Cool Tunnel" type. The top handguard and handguard cap are slotted to accept the piston system. This system is more reliable, cleaner, and runs cooler than other system. This system consists of: Gas block, adjustable gas plug, gas piston, gas piston spring, gas piston tube, one piece semi auto bolt carrier, bolt spring, M4 Cool Tunnel handguards and handguard cap. It comes assembled as shown with instructions. MADE IN USA!



Installing the Blackthorne Piston System (BPS)


  1. Ensure that the firearm is unloaded and there is no round in the chamber
  2. Remove the upper assembly from the lower assembly by drifting the two pins and lifting the upper off.
  3. Remove the bolt carrier group and charging handle.
  4. Remove the bolt from the bolt carrier
  5. Insert the bolt spring into the BPS carrier with the smaller diameter facing forward.
  6. Install your bolt assembly, cam pin, firing pin, and firing pin retaining pin into the BPS carrier, set aside this assembly.
  7. Remove the handguards
  8. Remove the flash hider (if so equipped) and front sight base with the gas tube still attached(see owners manual of your particular rifle for instructions on how to remove your front sight base), and handguard cap.
  9. Insert the BPS bolt carrier assembly and charging handle into the upper receiver and ensure that it is all the way forward.
  10. With your thumb against the rear of the Bolt carrier pushing forward, slide the handguard cap and BPS gas block with piston installed over the end of the barrel.  Carefully guide the rear of the gas piston through the delta ring (ensure that the delta ring, weld spring, and retaining ring gaps are lined up) and through the gas tube hole (cloverleaf shaped) in the upper.  Carefully feel for the rear end of the gas piston to contact the integral lug of the BPS bolt carrier.  IT IS EXTREMELY IMPORTANT THAT THE GAS PISTON CONTACTS THE LUG AND THAT THERE IS LESS THAN .005” SPACE BETWEEN THE TWO WHEN THE BOLT IS IN BATTERY (ALL THE WAY FORWARD). Ensure that the gas bleed hole in the gas block is aligned with the gas bleed hole in the barrel.
  11. Ensure that the gas block is “timed” correctly on the barrel (ie. That it is installed straight up) then tighten the two 8-32 clamp screws to 48 in/lbs. The BPS will align with the gas port of any mil spec CARBINE length barrel.
  12. Install the flash hider and handguards (note: The handguard with the cutout in the top front goes on top)


Function check


  1. Check to ensure that the bolt carrier group is fully in battery when stopped in its forward motion by the barrel extension.  Make ABSOLUTELY CERTAIN that the bolt goes fully into battery and is NOT stopped by the gas piston but is stopped in its FULLY LOCKED position by the barrel extension.
  2. Check to ensure that there is less than .005” between the gas piston and bolt carrier lug.  Ensure that the bolt carrier slides freely and locks into battery.




  1. The BPS is adjustable for gas pressure.  The gas plug has three positive detents marked I,  II,  III.  From the factory the gas plug is set at the I setting.  The I setting is the one used most of the time and sends the proper amount of gas necessary to reliably operate the firearm.  The II setting sends more gas to operate the weapon and should be used only when shooting reduced loads.  OPERATING THE FIREARM WITH FULL POWER LOADS AND THE GAS PLUG SET TO II CAN RESULT IN DAMAGE TO THE GAS PISTON!  Position III is a gas cutoff and allows for single shot and manual cycling of the action.
  2. To adjust the gas plug push down on the gas plug detent on the front of the gas block ( a small screwdriver or bullet tip can be used) and rotate the gas plug to the desired setting, release the gas plug detent and ensure that it positively locks the gas plug in position.




  1. Ensure that the rifle is on SAFE, unloaded and the chamber is empty.
  2. Remove the upper from the lower, remove the bolt carrier group and charging handle
  3. Depress the gas plug detent and unscrew the gas plug from the gas block, care should be taken as the gas plug is spring loaded.
  4. Clean the gas plug, the gas piston, and gas chamber with CLP or equivalent, then wipe dry.
  5. Reinstall parts ensuring that the gas plug is screwed down as far as possible and that the gas plug detent is in the I position.




  1. The gas block has an M1913 Picatinny standard rail on top, 1.200” height from centerline of bore.
  3. Position II of the gas plug should only be used with reduced loads, shooting full power loads with the gas plug in position II can result in damage to the gas piston.
  4. Most malfunctions are the result of improper ammunition.  Ensure that you are using new commercially manufactured domestically produced ammunition in good condition.  Also, ensure that the gas hole in the barrel lines up with the gas hole in the gas block when installed AND that there is less than .005” between the gas piston and bolt carrier lug.  This is not an issue with mil spec parts as the linear dimensions are correct but non-mil spec parts may not be the correct dimension.

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  • Model: AR-PISTON
  • 31 Units in Stock

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This product was added to our catalog on Wednesday 07 July, 2010.