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The Auxiliary Spring Rate Kit for the JP Silent Captured Spring allow custom tuning of the device to a given rifle. While the default spring installed in the JPSCS is selected to offer the best performance for the widest range of configurations, a simple disassembly and reassembly of the JPSCS with a slightly different spring rate can yield truly optimized performance, particularly for rifles without an adjustable gas block.
In selecting a spring for your rifle, install a heavier spring in you feel the buffer head fully compress and bottom out while firing the weapon. If the bolt carrier will not lock back reliably on the last round fired, even when completely gassed (in the case of an adjustable gas block), select a lighter spring.
In particular, if you have an ambidextrous bolt catch extension installed (e.g., the Magpul BAD lever), this extra mass on the bolt catch, in combination with the faster bolt velocity using the SCS with a heavier spring, may cause the bolt to ride over the catch before it has time to engage. In this case, a lighter spring should remedy the problem.
The included springs from lightest to heaviest are listed below with the color with which they are coded: