a constant - pressure hydraulic shock - absorbing device suitable for use as a recoil brake in a weapon system . by use of a piston that decreases in diameter as the applied force increases , a constant resistive force independent of the applied force is obtained . the varying diameter of the piston creates a varying aperture for the displacement of fluid by the piston , thus varying the resistive force . this results in minimum travel and / or minimum / maximum force transmitted through the shock absorber for a given applied energy .