AR-15 Buffer Weight, Spring Rate and Gas Length That Cycle Together

A correctly gassed AR-15 does four things, and all four have to be true at once. It locks the bolt back on an empty magazine every time, with any ammunition in its intended power range. It ejects to a consistent clock position. It strips and chambers the next round without the carrier arriving at the rear of the tube hard enough to be audible over the report. And it does all of that dirty, cold and dry, not just clean and freshly lubricated.

Reference condition for every recommendation on this page: 8.00 lb rifle as-fired, 5.56 NATO 55 gr at 3,100 fps from a 16 in barrel, 59°F at sea level, mil-spec bolt carrier group at 11.6 oz, standard buffer tube. Change any of those and the tune moves — a lightweight carrier at 8.5 oz behaves like about one buffer step lighter.

Ejection pattern is the measurement because it is the only one you can take without instrumentation. It is a proxy for carrier velocity and a decent one, but it is still a proxy — a rifle can eject at a beautiful 4 o'clock and be over-gassed if the extractor is doing too much of the work. Treat it as the first reading, not the only one.

Nominal masses vary by a tenth of an ounce between makers, and a few makers use their own naming that does not map to the mil-spec ladder. Weigh yours on a reloading scale if the tune is close — a kitchen scale will not resolve 0.8 oz reliably at this range.

The A5 system deserves a note because it is the most under-used fix on this list. It runs a rifle-length spring in a slightly longer carbine tube with a longer buffer, giving the carrier more travel and a flatter spring rate across that travel. The result is a noticeably softer impulse and a wider tuning window than the carbine system offers, at the cost of a proprietary tube and a receiver extension swap. For a rifle that has to run both suppressed and unsuppressed, it is the better architecture.

Buffer springs are consumable. A carbine spring cycled ten thousand times has taken a set, lost free length and is delivering measurably less force at the end of its stroke than it did new. That shows up as short-stroking which appears gradually and gets blamed on ammunition.

Measure free length against the specs below. A carbine spring under 10.0 in or a rifle spring under 12.5 in has taken enough set to replace, and springs cost less than a box of ammunition.

Spring rate and buffer mass are not interchangeable even though both slow the carrier. Mass changes the carrier's momentum through the whole cycle, including the forward stroke, so a heavy buffer can cause feeding problems on a weak magazine spring. A stiffer spring resists mostly at the end of the rearward stroke and pushes harder on the return. If a rifle short-strokes on the return — bolt stopping just shy of battery — you want less spring, not less buffer.

Gas length is the distance from chamber to gas port. Dwell is what is left between the port and the muzzle. Longer gas length means lower port pressure and a gentler impulse, and it also means less dwell, so there is a floor below which the system cannot cycle. Port diameters below are the ranges commonly cut in 5.56 barrels.

Mid-length gas on a 16 in barrel is the single best value decision in an AR-15 build. It lowers port pressure meaningfully against carbine gas at the same barrel length, which softens the impulse, reduces carrier velocity and widens the tuning window enough that one buffer will run the rifle suppressed and unsuppressed. Carbine gas on a 16 in barrel is a legacy of 14.5 in military barrels, and there is no engineering reason to prefer it.

Two hard constraints whatever the configuration. Leave at least 1.5 in of barrel past the gas port. And never pair rifle-length gas with a barrel under 18 in — the dwell is not there, and the rifle will short-stroke regardless of spring or buffer.

Six inputs: barrel length, gas system length, caliber, gas port diameter where you know it, suppressed or unsuppressed, and ammunition power factor. It returns a starting buffer mass in ounces and a spring specification.

Power factor is the input people skip. Commercial steel-case 55 gr runs meaningfully softer than 62 gr NATO-spec, and a rifle tuned on one will sit a step out on the other. Enter the ammunition you actually shoot most, and expect everything else to land an hour off on the clock.

The output is a starting point, not a verdict. Fit it, shoot twenty rounds, read the brass, and adjust from the ejection table below. Any tuning guidance that does not end in a range trip is guessing.

• ▸Know your port diameter if you can. Pin gauges in 0.001 in steps from .060 to .100 will tell you in a minute and take all the guesswork out of the process.
• ▸Weigh your bolt carrier group. Mil-spec is about 11.6 oz; lightweight carriers run 8.5 to 10 oz and behave like a lighter buffer.
• ▸Note whether you have an adjustable gas block. If you do, tune there first and leave the buffer at H.
• ▸Test cold and dirty, not just clean. A rifle that only cycles when freshly lubricated is not tuned, it is lucky.

Stand the rifle up, fire five rounds, and note where the brass lands relative to the muzzle as twelve o'clock. Do it on a flat surface where you can see the pile. Five rounds is a pattern; one round is noise.

Two cautions on this method, because it gets over-trusted. Ejection angle is influenced by extractor tension, ejector spring strength and case material as well as by carrier velocity, so steel-case ammunition often lands an hour further forward than brass out of the same rifle. And a rifle ejecting at 3 o'clock with a suppressor and 5 o'clock without one is not two rifles out of tune — it is one correctly tuned rifle, working in both conditions.

The 0.8 oz per hour figure is a working rule rather than a measured constant. It holds reasonably well within one or two hours of a correct tune, and progressively less well outside that.

Work down this list. Most of it costs nothing, and the parts that cost money are at the bottom for a reason.

• ▸Check the gas block alignment. A block rotated a few degrees off, or slid forward of the port, is the most common cause of a rifle that short-strokes from new. Pull the handguard and look.
• ▸Check the gas tube is seated in the carrier key and that the key screws are staked. Loose key screws leak gas and produce intermittent short-stroking that comes and goes.
• ▸Measure the spring's free length. Under 10.0 in on a carbine spring, replace it.
• ▸Check for carbon lock in the gas block, particularly on a suppressed rifle. A partially blocked port behaves exactly like an undersized one.
• ▸Replace the extractor spring and insert. Cheap, five minutes, and it eliminates the most common cause of erratic ejection that gets misdiagnosed as gas.
• ▸Only now change the buffer, one step at a time, five rounds between changes.
• ▸If none of that works, measure the port with pin gauges and compare against the table above. A port outside spec needs a gunsmith or a new barrel, not a heavier buffer.

A buffer changes reciprocating mass. That is a blunt instrument applied to the end of a chain whose first link is a hole drilled in a barrel, and it can only correct a problem that was small to begin with.

Consider a 16 in barrel with a .0935 in rifle-gas port cut into it by mistake instead of a .0700 carbine port. Port area is nearly double. No buffer in production absorbs that — you would be running an H3 and still battering the receiver, and the rifle would beat itself apart over a few thousand rounds while technically functioning. The answer there is an adjustable gas block or a new barrel, and any amount of buffer swapping is money spent avoiding that conclusion.

The corollary is worth stating plainly. An adjustable gas block on a build you intend to suppress is not a luxury item. It is the component that makes the rest of the tune possible, and it costs about what two buffers cost when you buy them in sequence. Fit one at build time and the buffer question mostly answers itself at H.