Quick answer

Replacing an AR15 buffer tube is a 20–30 minute job if you match the tube to your stock and receiver. Use a mil-spec tube for mil-spec stocks, torque the castle nut to about 40 inch-pounds, and stake or thread-lock it so it cannot back out. The Star Tactical Supply AR15 Mil-Spec Buffer Tube in Blue is a direct replacement for standard carbine setups.

Mil-spec vs commercial: the one measurement that matters

Buffer tubes look alike, but the outer diameter and thread pitch decide which stocks fit. Mil-spec tubes use a slightly smaller diameter and a different thread than commercial tubes. If you install the wrong one, your stock will either wobble or refuse to slide on. Measure your current tube or check your stock's spec sheet before ordering. The AR15 Mil-Spec Buffer Tube Blue from Star Tactical Supply is a mil-spec-style tube for AR-15 platforms, so it pairs with mil-spec carbine stocks.

Tools and parts to gather first

You need a castle nut wrench, a vice block or reaction rod, a torque wrench that reads inch-pounds, and a small punch for staking. A crush washer is not used on the buffer tube, but keep one on hand if you also swap the muzzle device. The AR15 M16 M4 A2 TPI Birdcage Muzzle Brake Flash Hider 1/2x28 includes a crush washer and fits 1/2x28 threads if you are doing both jobs at once. Work on a bench with the upper removed and the lower secured.

Step-by-step replacement

Follow this order to avoid cross-threading or a loose stock. Each step includes the mistake that most often causes problems.

After the install: function check

Once the tube is tight, slide the stock on and cycle the action by hand. The stock should move smoothly without wobble, and the buffer should travel without binding. If the stock binds, the tube may be commercial-spec or the receiver extension may be out of spec. If the buffer binds, check that the buffer retainer and spring are seated correctly. A quick function check now prevents a stuck stock at the range.

Install steps with checkpoints

  1. Secure the lower receiver: Clamp the lower in a vice block or use a reaction rod so the receiver cannot twist. Do not clamp directly on the buffer tube threads. Checkpoint: the lower should not move when you push on the receiver extension.
  2. Remove the old buffer tube: Loosen the castle nut with a castle nut wrench. If it is staked, cut the stake with a small drill bit or punch before turning. Unscrew the tube by hand once the nut is free. Mistake to avoid: forcing a staked nut can damage the receiver threads.
  3. Inspect and clean threads: Wipe the receiver threads and the new tube threads with a clean rag. Look for burrs or crossed threads. Apply a small amount of grease or anti-seize to the tube threads. Checkpoint: threads should look sharp and uniform. If they are damaged, stop and have a gunsmith chase them.
  4. Install the new buffer tube: Thread the tube in by hand until it stops. The buffer retainer pin should sit in the notch at the bottom of the tube. Do not use a wrench to force it past hand-tight. Mistake to avoid: cross-threading. If it binds, back it out and start again.
  5. Tighten the castle nut: Hold the tube in place and tighten the castle nut with a torque wrench to about 40 inch-pounds. This is the buffer tube torque spec for most AR-15 receivers. Checkpoint: the tube should not rotate when you try to turn it by hand.
  6. Stake or secure the castle nut: Use a center punch to stake the castle nut into the receiver notch, or apply removable thread-locker if you prefer not to stake. Staking is the mil-spec method. Mistake to avoid: skipping this step. An unstaked castle nut can back out under recoil.
  7. Reinstall the stock and buffer: Slide the stock onto the tube and reinstall the buffer and spring. Test the stock adjustment and ensure the buffer moves freely. Checkpoint: the stock should lock in each position without wobble.

Parts that fit this job

  • AR15 Mil-Spec Buffer Tube Blue: AR15 Mil-Spec Buffer Tube in Blue, Star Tactical Supply brand, mil-spec style for AR-15 platforms. A mil-spec tube is the correct match for mil-spec carbine stocks and standard AR-15 lower receivers.
  • AR15 M16 M4 A2 TPI Birdcage Muzzle Brake Flash Hider 1/2x28: AR15 M16 M4 A2 TPI Birdcage Muzzle Brake Flash Hider with 1/2x28 threading, high grade steel, includes crush washer, made in USA. If you are already working on the upper, this muzzle device fits standard 1/2x28 threads and comes with the washer you need.
  • AR15 7" Slim Keymod Handguard Rail One Piece Free Float with 6 Screws: AR15 7" Slim Keymod Handguard Rail One Piece Free Float with 6 screws, includes steel barrel nut, lightweight 6061-T6 aluminum, Allen key and crush washers included. A free-float handguard can be installed during the same bench session, but it requires removing the barrel nut, so plan the order of work.