The Cavity Spring is the the RCG's equivalent of a BCG's buffer spring.

The Cavity Spring is Resolute Defense's innovation which makes condensing of the operating system from the buffer tube into the upper receiver possible. This innovative spring solves 2 problems which has prevented similar efforts from delivering reliable performance. 1. it allows a spring with the necessary forces to operate without preventing the hammer to access the firing pin. 2. due to the coil designs it allows the springs solid height to be reduced by 50% as compared to an equivalently sized coil spring, while offering greater compression forces. 

Material: .059" 17-7 PH Stainless Steel