AR-15 Buffer Tube

AR-15 Buffer Tube

The buffer tube — also called a 'receiver extension' — is the tubular component at the rear of an AR-15 lower receiver that houses the recoil buffer and spring, and onto which the stock is mounted. It's critical to how the rifle cycles, and the wrong combination of buffer tube length + buffer weight + gas system causes most AR-15 cycling problems.

What the buffer tube does

After a round fires, hot gas pushes the bolt carrier rearward. The carrier travels back into the buffer tube, compressing the recoil spring (and the buffer in front of it) until it stops. The compressed spring then drives the carrier forward, stripping a fresh round from the magazine and locking the bolt back into the barrel extension.

The buffer tube is the chamber where this happens. The buffer weight controls how fast the carrier travels rearward — heavier buffers slow the carrier, which is what you want when the rifle is over-gassed (excess gas pressure causing the bolt to move too violently).

Mil-spec vs commercial — at a glance

	Mil-spec	Commercial
Outer diameter	1.148"	1.168"
Manufacturing	Rolled forge (cold-rolled)	Extruded
End angle	Flat (90°)	Sloped
Stock compatibility	Mil-spec stocks only	Commercial stocks only
Quality / strength	Same once anodized	Same
Cost	$30-60	$20-40

Bottom line: get mil-spec. The stock ecosystem is dramatically wider (Magpul CTR, BCM Gunfighter, B5 Bravo, ARFCOM-favored 'Sopmod' stocks are all mil-spec). Commercial is a budget compromise; the cost savings don't justify the limited stock selection.

Buffer weights — what's in the tube

Buffer	Weight	Use case
Standard carbine (C)	3.0 oz	16" carbine-length gas, mid-pressure
H	3.8 oz	16" carbine + mid-length gas; standard for quality M4-pattern
H2	4.6-4.7 oz	Over-gassed rifles, suppressed shooting, 14.5" barrels
H3	5.4-5.6 oz	SBR / 5.56 pistol builds with short gas systems
Rifle	5.2 oz	A1/A2 fixed-stock rifles (different physical length)
Tungsten / heavy	5.0+ oz	Specialty — very-over-gassed builds, certain piston systems

Diagnosing cycling problems via buffer + brass

Brass ejection pattern is the diagnostic tool. Stand a few quality AR shooters in line at a range and watch where their brass lands:

4-5 o'clock, 8-12 feet — properly gassed, buffer weight is right. Target zone.
3 o'clock, 15+ feet (or in your face) — over-gassed and/or buffer too light. Try H2 or H3, or adjustable gas block.
5-6 o'clock, < 5 feet — under-gassed and/or buffer too heavy. Try lighter buffer or open the gas port.
Failure to feed — buffer too heavy for available gas. Drop a weight class.
Brass-to-the-face — almost always over-gassed, short-stroking the ejector. Heavier buffer or adjustable gas block.

What to buy

Tube — BCM, KAC, Geissele, Vltor, Aero Precision. Mil-spec. Type III hardcoat anodized.
Castle nut + end plate — usually included with the tube. Stake the castle nut after final torque (35-39 ft-lb).
Buffer — Vltor A5, Spike's Tactical, Geissele Super 42 (combo spring + buffer). H, H2, H3 as needed.
Spring — standard carbine spring is fine; chrome silicon (CS) springs (Sprinco, Tubb, Geissele Super 42) are smoother and longer-lived.

Frequently asked

What's the difference between mil-spec and commercial buffer tubes?

Outer diameter. Mil-spec is 1.148 inches (rolled-forge process). Commercial is 1.168 inches (extruded process, slightly cheaper to make). Mil-spec is the standard for nearly all quality AR-15s — stocks from the major makers (Magpul, B5, BCM) ship in both flavors but mil-spec is the default. The functional difference is zero; the practical difference is that you can't mix-and-match — a commercial stock won't fit a mil-spec tube and vice versa.

Carbine vs rifle vs pistol buffer tubes?

Three lengths. Carbine length (the standard collapsible-stock AR-15) accepts a 3-3.8 oz buffer and a carbine recoil spring. Rifle length (fixed-stock A1/A2 rifles) is longer, accepts a heavier 5.2 oz rifle buffer. Pistol buffer tube has no provisions for a stock — it's just a receiver extension to hold the buffer and spring, designed for AR pistol builds where a stock would make it an SBR. The pistol tube + a 'brace' was the SB Tactical / 2023 ATF brace rule territory.

What buffer weight should I use?

Standard carbine buffer (3.0 oz, marked 'C') for 16-inch carbine-length gas. H buffer (3.8 oz) for 14.5-inch + mid-length gas with M4 feed ramps. H2 (4.6-4.7 oz) for over-gassed rifles or suppressed shooting. H3 (5.4-5.6 oz) for heavily over-gassed or 5.56 pistol builds with very short gas systems. Wrong buffer = either short-stroking (light buffer + over-gassed = brass to the face, fast cycling) or failure to feed (heavy buffer + under-gassed). Diagnose with brass ejection pattern: 3 o'clock = right; 4-5 o'clock = good; past 5 o'clock = light buffer or over-gassed.

Is a pistol buffer tube legal?

Pistol buffer tubes themselves are legal. The legal question is whether you have a stock or brace attached, and how the configuration is classified by the ATF. As of the 2023 brace rule (which has been in legal flux in some circuits), pistols configured with certain braces may be considered SBRs requiring NFA registration. The pistol buffer tube alone — bare, no brace, no stock — is unambiguously legal. Consult a firearms attorney for brace-configuration questions.

What length buffer tube do I need for my AR?

Match to your gas system + barrel length. 16+ inch barrel with carbine-length gas → standard carbine buffer tube + H buffer. 14.5-16 inch with mid-length gas → same, possibly H. 16+ inch with rifle-length gas → consider a rifle buffer tube + rifle buffer. 11.5-12.5 inch SBR/pistol → carbine tube + H2 or H3 buffer + adjustable gas block recommended. The cycling reliability depends on matching buffer weight + spring + gas port to your specific configuration.

Do I need an adjustable gas block?

Not for a standard 16-inch carbine-length gas. Highly recommended for SBR/pistol builds, suppressed shooting, or anything with a non-standard gas system. The adjustable gas block lets you tune the gas pressure (and thus bolt-carrier velocity) instead of fighting it with buffer weight changes. SLR Rifleworks and JP Enterprises make the most-respected options.