AR-15 Buffer Spring & Buffer
The buffer spring works with the buffer weight to control bolt carrier group velocity. A worn buffer spring causes bolt over-travel, increased felt recoil, and reliability issues. Replace every 5,000–10,000 rounds. The standard carbine buffer spring length is 11.75"; if yours measures under 11", replace it.
Buffer weights control cycling speed. Standard carbine buffer: 3 oz. Heavy buffer (H): 3.8 oz — reduces felt recoil and helps with overgassed rifles. H2: 4.6 oz — for suppressed or pistol-length gas builds. Sprinco, Wolff, and BCM all make quality replacement springs ($8–$15).
Lower Parts Kit Small Parts
The lower parts kit (LPK) contains all the small parts needed to complete an AR-15 lower: trigger guard, magazine catch, bolt catch, safety selector, pistol grip screw, and all associated springs, pins and detents. Keep a spare LPK on hand — these parts are inexpensive and the most commonly lost during assembly.
| Part | Function | Replace When | Price |
|---|---|---|---|
| Detent pins | Retain pivot and takedown pins | If lost or damaged | $0.50 each |
| Extractor spring | Powers the extractor | Every 5,000–10,000 rounds | ~$3 |
| Gas rings (bolt) | Seal gas in the bolt carrier | When all 3 gaps align | ~$5/set |
| Buffer spring | Returns BCG to battery | Every 5,000–10,000 rounds | ~$10 |
| Firing pin retaining pin | Retains firing pin in BCG | If mushroomed or bent | ~$2 |
Roll Pins
Roll pins secure components like the bolt catch, trigger guard, and forward assist. They are single-use — once driven out, replace with a new pin. Always use a roll pin starter punch and a bench block to avoid damaging the receiver. Brownells sells replacement roll pin kits for the AR-15 for ~$8 (includes all sizes).