SKU: 100045016
UPC: 785939517521
MPN: RPBUFFERRETAINE
The Sons of Liberty Gun Works buffer retainer spring is the small coil that sits under the buffer retainer pin in the lower receiver, holding the detent up so it captures the buffer and keeps the bolt carrier group seated when you pop the upper open. It's a wear-and-loss part more than a wear-and-fail part, which is why builders and armorers keep spares on hand. If you're assembling a lower or chasing down a buffer that won't stay retained, this mil-spec spring drops into the standard receiver pocket on AR-15 and M4-pattern platforms. Browse our rifle parts for the rest of the lower build.
This is a coil compression spring built to mil-spec dimensions, paired with the buffer retainer pin that protrudes through the receiver floor at the rear of the magazine well. Under spring tension, the retainer rides up against the rear of the buffer and stops it from walking out when the upper is separated from the lower. Per manufacturer specifications the spring measures roughly 1 inch in length and weighs 0.1 lbs, which is consistent with standard AR-15 buffer retainer geometry. Because it works against the buffer body and not the action spring, length and free height matter — a short or set spring lets the detent drop below the receiver floor and the buffer pops loose under recoil. Pair it with a fresh retainer pin if yours shows mushrooming.
You'll reach for this spring during a lower receiver build, after losing the original to the launch that every AR-15 owner has experienced at least once, or when an older buffer starts slipping past a tired detent. Installation means dropping the spring into its pocket, setting the retainer pin on top, and compressing both with the buffer tube as you thread it on — keep a thumb over the detent so it doesn't go flying. Keep one in the spares kit alongside takedown and pivot pin springs. See our takedown pin and our recoil buffer pin to round out the small-parts drawer.
It sits beneath the buffer retainer pin in the lower receiver and keeps that pin pushed up against the rear of the buffer. The tension holds the buffer in place so the bolt carrier group stays seated when you separate the upper from the lower receiver.
Yes. This is a mil-spec spring sized for standard AR-15 and M4-pattern lower receiver buffer retainer pockets. It works with carbine and rifle buffer systems that use the conventional retainer pin and detent setup.
Drop the spring into its pocket at the rear of the magazine well, set the retainer pin on top, then thread the buffer tube on while pressing the pin down with your thumb until the tube edge holds it. Releasing it before the tube captures it is how most people launch the part across the room.
If the pin shows mushrooming on the top edge or no longer holds the buffer firmly, replace both together. A worn pin combined with a set spring is the usual reason a buffer slips loose under recoil.
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