The Alternative Spring Pack Kits are a pack of 5 different spring weights for the JP Silent Captured Spring that allow custom tuning of the device to a given rifle. While the default spring offers the best performance for the widest range of configurations, a simple disassembly and reassembly of the JPSCS-15 with a slightly different spring rate can yield truly optimized performance, particularly for rifles without an adjustable gas block. Mfg: J P Enterprises