Buffer Weights, Springs and Stocks: Making the Back Half of the Rifle Run

The buffer tube, buffer and spring are the parts of an AR-15 that nobody thinks about until the rifle stops working. They are also the cheapest fix for most cycling problems and the most common cause of them when chosen badly. The buffer's weight decides how fast the bolt carrier travels and how hard it hits at both ends of its stroke, and the tube's diameter decides which stocks fit. This guide explains the system and gives the combinations that work for the common builds.

How the system works

When the rifle fires, gas pushes the bolt carrier back into the buffer tube, compressing the spring and pushing the buffer ahead of it. The spring then drives everything forward to chamber the next round. The buffer's mass slows the carrier's rearward travel, delaying the unlock slightly and taming the impact against the rear of the tube, and its weight comes from steel or tungsten slugs inside a hollow body that shift under recoil and reduce bounce. Change the weight and you change the rifle's timing; the goal is a carrier that travels far enough to lock the bolt back on an empty magazine and no further.

The standard buffer weights

Carbine, three ounces: the original, right for a sixteen-inch carbine-gas rifle with a standard gas port.
H, three point eight ounces: one tungsten slug; the best all-round choice for sixteen-inch carbines, especially with a suppressor or a generous gas port.
H2, four point six ounces: two tungsten slugs; for over-gassed carbines, short-barrelled pistols and suppressed guns.
H3, five point four ounces: three tungsten slugs; for heavily over-gassed builds and .308 AR pistols.
Rifle buffer, five point two ounces in a longer body: for rifle-length A2 stocks only, with the rifle spring.

Heavier is not better. A buffer too heavy for the gas the rifle makes causes short stroking: the carrier does not travel far enough to pick up the next round or lock back. A buffer too light for an over-gassed rifle lets the carrier slam the tube, throws brass forward and hard, and wears the extractor and the cam pin. Start with an H buffer on a sixteen-inch rifle and adjust from the brass.

Reading the brass

With a standard rifle, cases should land between three and four o'clock, a few feet away. Brass thrown forward to one or two o'clock and far away means over-gassed: go up a buffer weight. Brass dribbling at five or six o'clock, failures to lock back on an empty magazine, or the bolt failing to strip a round means under-gassed or too heavy a buffer: go down a weight, then check the gas block alignment and the gas rings before anything else.

Springs

A standard carbine spring is cheap and right for almost every build. Flat-wire springs last longer and are a little smoother; braided and premium springs make small differences at high cost. Replace a spring every few thousand rounds or when it is noticeably shorter than a new one. Rifle-length springs are longer and only fit rifle-length tubes; never mix.

Mil-spec and commercial tubes

Carbine buffer tubes come in two diameters. A mil-spec tube measures about 1.146 inches and has rolled threads; a commercial tube measures about 1.168 inches and is usually cut with a sloped rear. Stocks are made for one or the other and a commercial stock on a mil-spec tube rattles while a mil-spec stock will not slide onto a commercial tube. Nearly all quality stocks are now mil-spec; buy a mil-spec tube unless you have a specific commercial stock to fit. Pistol tubes have no stock and are sized for braces; check the brace maker's fitment.

Matching to barrel and gas length

Sixteen-inch barrel, carbine gas: H buffer, sometimes carbine.
Sixteen-inch barrel, mid-length gas: carbine or H; the gentlest combination in the catalogue.
Fourteen-and-a-half inch pinned, carbine gas: H or H2.
Ten-and-a-half or eleven-and-a-half inch pistol, carbine gas: H2, sometimes H3.
.300 Blackout pistols with supersonic and subsonic ammunition: H2 and an adjustable gas block to run both.
Twenty-inch rifle with rifle gas and an A2 stock: rifle buffer and rifle spring.

Stocks

A collapsible stock does three things: sets the length of pull, gives the cheek a place, and holds the buffer tube's end. The basic six-position stock that ships on most rifles works. Upgrades buy a better cheek weld, a rubber butt pad that stays on the shoulder, storage for batteries, and less rattle. Fixed stocks are lighter and more rigid and suit a rifle that is never shared; a fixed stock on a carbine tube needs a spacer or a specific model. Length of pull with the stock fully collapsed suits shooting in armour or a heavy coat; two or three clicks out suits most people in a shirt.

Adjustable gas blocks and heavy carriers

The buffer is one of three places to tune a rifle's timing. An adjustable gas block at the other end lets you reduce the gas rather than fighting it with buffer weight, which is the cleanest fix for a suppressed or short-barrelled build and lets one rifle run subsonic and supersonic ammunition with a turn of a screw. A heavier bolt carrier does part of what a heavier buffer does with the mass in the upper instead. Most builds need none of this: a sixteen-inch mid-length rifle with an H buffer and a standard carrier is the combination that simply works.

Braces and pistol buffers

Pistol buffer tubes are shorter and often smooth, sized for a specific brace, and take the same carbine buffer and spring as a rifle tube. Some braces attach to a standard mil-spec carbine tube and some need their own; the brace maker's fitment page decides. Pistol builds run on H2 buffers more often than not because pistol-length gas systems are over-gassed by design, and a pistol that throws brass forward and batters the tube is asking for one.

Castle nuts and staking

The castle nut holds the tube, and a loose one is the most common reason a rifle that ran fine starts double-feeding. Torque it to about forty foot-pounds and stake the end plate into two of the nut's notches with a centre punch. A staked nut does not move; an unstaked one eventually does.

Read the parts kit guide for fitting the tube, the rifle-or-pistol guide before choosing a tube, and browse buffers, stocks and lowers in the catalogue.

General information, not legal advice. Current as of September 2026. Laws change; check the official source before you act.