ADVANTAGES: XP 5-Coil spring has a heavier spring load than OEM springs, thus negating the need for a D-Fender(TM) ring or adding CRANE Viton O-Rings Use 5-Coil spring with either black extractor spring insert OR CRANE Viton O-Rings, but not both Enhanced 4-Coil springs maintain an extended duty cycle load of a NEW Mil-Spec spring and is recommended for use in 18""+BBL lengths for those who retrieve their brass for reloading While the XP 5-Coil spring is totally reliable for all 5.56 BBL lengths, it can be aggressive on case rims in 18""+length BBL rifles, and may be too strong for some bolts with extremely tight bolt face tolerances Mfg: Sprinco Usa