AR-15 Buffer Tubes & Buffer Weights Explained
The buffer tube — officially the receiver extension — is the cylindrical aluminum housing attached to the rear of an AR-15 lower receiver. It contains the buffer (a weighted slug) and the buffer spring (the recoil spring). Together, they absorb the bolt carrier group's rearward travel after firing and return it to battery. The buffer tube also serves as the mounting point for the buttstock or pistol brace.
Mil-Spec vs Commercial buffer tubes: The most important decision when selecting a buffer tube. Mil-spec tubes have an outer diameter of 1.146 inches and are forged from 7075-T6 aluminum with rolled threads. Commercial tubes measure 1.17 inches in diameter, are made from 6061-T6 aluminum with cut threads, and have a slightly angled rear face. These dimensions are not interchangeable — a commercial stock will be loose on a mil-spec tube, and a mil-spec stock cannot be forced onto a commercial tube without damage. Virtually every quality aftermarket stock from Magpul, B5 Systems, and BCM is designed for mil-spec tubes. Purchase mil-spec unless there is a specific reason otherwise.
Buffer weight selection: Standard carbine buffers weigh approximately 3 ounces. The H buffer runs 3.8 ounces, H2 at 4.6 ounces, H3 at 5.4 ounces. Brass ejecting at 12 o'clock or forward of 3-4 o'clock indicates over-gassing — step up one buffer weight before tuning the gas block. Suppressor use generally requires moving up one to two buffer weights to compensate for increased backpressure.
Pistol buffer tubes are shorter, have no adjustment notches, and are designed specifically for AR pistol builds. They are not interchangeable with carbine or rifle buffer tubes. For additional lower build guidance, see our mil-spec vs upgraded trigger comparison and comparison guides hub.
Buffer and Buffer Tube Frequently Asked Questions
What is the difference between mil-spec and commercial buffer tubes?
The critical difference is outer diameter. Mil-spec buffer tubes measure 1.146 inches and are forged from 7075-T6 aluminum with rolled threads. Commercial tubes measure 1.17 inches, are made from 6061-T6 aluminum with cut threads, and have a slightly angled rear face. Stocks designed for one spec are not reliably compatible with the other — a commercial stock will wobble on a mil-spec tube, and a mil-spec stock cannot be forced onto a commercial tube without damage. Most quality aftermarket stocks are built to mil-spec dimensions.
How do I know which buffer weight I need?
Start with a standard carbine buffer (3 oz) and observe your brass ejection pattern. Brass ejecting between 3 and 5 o'clock indicates a well-tuned system. Brass ejecting at 12 o'clock or forward indicates over-gassing — step up to an H buffer (3.8 oz) and re-evaluate. Brass ejecting straight back or the bolt failing to lock back on empty indicates under-gassing — try a lighter buffer or open the gas block slightly. Suppressor use generally requires moving up one to two buffer weights to compensate for increased backpressure.
What is the H2 buffer and when should I use it?
The H2 buffer weighs 4.6 ounces — between the H (3.8 oz) and H3 (5.4 oz). It is commonly used on carbine-length gas systems running heavier loads, on barrels 14.5 inches and shorter that tend to run overgassed, and on rifles used with suppressors. If the standard H buffer still produces a snappy recoil impulse or brass ejecting forward of the 3-4 o'clock position, the H2 is the next logical step before adjusting the gas block.
Can I use any buffer and spring in either a mil-spec or commercial tube?
Yes — the interior dimensions of mil-spec and commercial buffer tubes are identical. Buffers and buffer springs are interchangeable between both tube types. The only compatibility concern is between carbine and rifle-length systems — rifle buffer tubes are longer and use a longer buffer and spring that is not interchangeable with carbine components.
What buffer tube do I need for an AR pistol?
AR pistol builds require a pistol buffer tube — shorter than a standard carbine tube, without adjustment notches, and designed for pistol braces rather than traditional stocks. Pistol buffer tubes are not interchangeable with carbine or rifle tubes. Confirm the lower includes a pistol buffer tube to maintain the correct pistol classification under federal law.