The Armaspec Gen3 Sound Mitigation Buffer SMB is a single-stage captured drop-in replacement for your standard buffer and spring. The SMB is designed to keep the buffer spring from rubbing against the inner wall of the buffer tube providing smooth, quiet operation. FeaturesSingle-stage captured drop-in replacement for your standard buffer and springReduce felt recoilKeep the buffer spring from rubbing against the inner wall of the buffer tube providing smooth and quiet operationAdjustable spring weight (*Main spring can be trimmed at .25 lbs per coil to reduce tension up to a max of 12 coils)Note: For proper function of the SMB-9 in your 9mm rifle or pistol, the extra weight on the back of the bolt carrier needs to be removed. It is held in with a spring pin.