AR-15 / .308 6-Position Mil-Spec Carbine Buffer Tube | Receiver Extension | SKU: 100028013

Build it right from the backbone. This AR-15 / .308 6-Position Mil-Spec Carbine Buffer Tube (receiver extension) gives your rifle or carbine a rock-solid foundation for adjustable stocks, smooth cycling, and reliable lock-up. Precision machined to mil-spec outside diameter (≈1.148") with six indexed positions, it accepts any mil-spec carbine stock and installs on standard AR lower receivers with the common 1-3/16"-16 threads. Whether you’re finishing a lightweight 5.56 build or tuning a hard-use .308 gas gun, this tube delivers the fit, feel, and durability you expect.

Why This Mil-Spec Buffer Tube

• True mil-spec diameter: Compatible with mil-spec carbine stocks (1.148" OD). Avoids the wobble and fit issues common to commercial-spec tubes (≈1.168").

• Six positive lock positions: Dial in length of pull for armor, layers, or prone/standing transitions—then lock with crisp detents.

• Smooth cycling: Uniform internal finish promotes consistent bolt travel and reduced friction for reliable feeding and ejection.

• Hard, wear-resistant finish: Anodized aluminum resists scuffs, corrosion, and the repeated impacts of the buffer system.

• AR-15 & .308 versatility: Works on AR-15/M4 lowers and .308/AR-10-pattern lowers (DPMS/SR-25 pattern) when paired with the correct buffer and spring for your caliber.

Key Features at a Glance

• Platform Fit: AR-15/M4 and .308 AR (DPMS/SR-25 pattern)*

• Spec: Mil-spec carbine receiver extension (≈1.148" OD)

• Adjustment: 6 positions (indexed)

• Thread: 1-3/16"-16 (standard AR lower receiver)

• Material: Durable, hard-anodized aluminum

• Finish: Matte black, non-reflective

• Compatibility: For mil-spec carbine stocks only (not commercial-spec)

• SKU: 100028013

*Note: External tube spec is the same; buffers and springs differ between AR-15 and .308 builds.

What It Works With

• Stocks: Any mil-spec carbine stock (Magpul, B5, BCM, etc.)

• Calibers (AR-15): 5.56/.223, .300 BLK, 6mm ARC, 6.5 Grendel, 7.62×39, and more

• Calibers (.308 AR): 7.62×51/.308 Win, 6.5 Creedmoor gas guns, .243 Win, etc.

• Buffers/Springs:

  • AR-15 carbine buffer & spring for AR-15 builds

  • .308 carbine buffer & spring (shorter buffer, heavier mass) for .308 builds

Installation Tips (Gunsmith-Level Finish at Home)

1. Prep the lower: Verify clear. Remove the old end plate/castle nut if replacing.

2. Align detents: Transfer the rear takedown detent & spring and buffer retainer & spring correctly.

3. Thread the tube: Spin in until the tube just retains the buffer retainer.

4. Castle nut torque: Tighten the castle nut to ~40 ft-lb (common spec; follow your OEM guidance).

5. Stake it: Stake the castle nut into the end plate to resist loosening under recoil.

6. Function check: Ensure the buffer, spring, and BCG move smoothly and the stock locks into all six positions.

Pro Tip: If you’re running a suppressor or unusual gas setup, consider an adjustable gas block and buffer weight tuning (H1/H2/H3 or .308 equivalents) to optimize recoil impulse.

AR-15 vs. .308: What to Know

• The tube’s outer spec is shared, but internal system parts differ.

• Do not mix AR-15 carbine buffers/springs with .308 systems (and vice versa).

• Some .308 manufacturers specify unique buffer lengths/weights—follow your receiver manufacturer’s spec for best results.

Who It’s For

• Builders & armorers who want repeatable, mil-spec fitment

• Duty & defensive carbines where castle-nut staking and reliability matter

• Competition & training rifles that need precise length-of-pull adjustments

• Hunting & field carbines where durability, corrosion resistance, and weight balance count

Maintenance

• Wipe the tube interior lightly with oil during cleaning; avoid heavy grease that can trap grit.

• Periodically check castle-nut staking and end-plate alignment after high round counts.

• Replace worn buffers/springs on interval to maintain timing and reliability.

Specifications

SEO Highlights (for Merchandising & Search)

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Start your build with the part that sets the standard—this AR-15/.308 6-Position Mil-Spec Carbine Buffer Tube delivers rock-solid stock lock-up, smooth cycling, and proven durability from the first round to the last.
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