The Armaspec Stealth Recoil Spring (SRS) Gen 4 is a captured, one-piece drop-in replacement for the standard AR-15 carbine buffer and action spring stack. A multi-stage spring inside a sealed housing tunes recoil impulse smoother than a factory H1/H2 buffer while eliminating the tinny spring-slap you get from an unshrouded action spring - the entire recoil stroke runs quiet. Specs & features: Captured one-piece design replaces both buffer and action spring H4 weight - heavier than factory carbine (H1), tunes hot loads and shorter gas systems Multi-stage internal spring softens felt recoil and impulse Sealed housing eliminates spring rattle and noise Drop-in install - no tuning, no shims Compatible with standard AR-15 carbine-length receiver extensions Fitment: AR-15 with a carbine-length buffer tube (7075-T6 or 6061 aluminum). Not for rifle-length tubes or AR-10 platforms. If you are running a suppressor, adjustable gas block, or a 10.5"-14.5" barrel with a carbine gas system, the H4 weight will help slow the bolt carrier speed for cleaner cycling.