One of the most significant features of the USP is the mechanical recoil reduction system. This system is incorporated into the recoil spring assembly, located below the barrel and consists of a heavy, captive coil spring around the guide rod. Designed primarily to buffer the slide and barrel and reduce recoil effects on the pistol components, the system also lowers the recoil forces felt by the shooter up to 30%. The USP recoil reduction system is insensitive to ammunition types and does not require adjustment or maintenance. It functions effectively in all USP models. Using this same recoil reduction system, one of the related H&K Mk 23 .45 ACP pistols fired more than 30,000 high pressure (+P) cartridges and 6,000 proof loads without damage or excessive wear to any major components. Abuse and function-testing of USPs have seen more than 20,000 rounds of .40 S&W fired without a component failure. Milspec environmental tests were conducted in high and low temperatures, in mud, immersed in water and in salt spray. In one particular test, a bullet was deliberately lodged in the barrel and another bullet was fired to clear the obstruction. The barrel was successfully cleared with only minor structural deformation and continued to produce consistent groups when test fired for accuracy.