“BLUE Spring” ENHANCED Power Buffer Spring, Offers improved performance with most non-suppressed 12.5″-14.5″ SBR’s, 16″ DI Mid-Length BBLS, and all mid-length uppers with adjustable gas blocks. Best performance is usually achieved using buffers ranging from ST-2T, H2, DPMS Extra Heavy Buffer. Will run in 18″+ Length BBL RRA’S with ST-T2 Buffers. Color Coded BLUE.