Heckler & Koch does not authorize caliber conversions, but a separate barrel for a .357 Sig or .40 S&W can be purchased from HK, or from companies like Midway USA or Jarvis, Inc. EFK Fire Dragon, an Arizona based pistol accessories company produces conversion barrels for both Full Size and Compact USP models. These conversion barrels can convert a .40 S&W to .357 Sig, and vice versa. They also manufacture 9mm conversion barrels. These 9mm conversion barrels can convert a 40/357Sig to a 9mm
, but not a 9mm to a .40/.357 Sig. Also, a more powerful recoil spring may be need for a .357 Sig conversion as the .357 Sig operates at a higher PSI. Heckler & Koch currently does not make the Full Size USP in .357 Sig, making the .357 Sig conversion for the Full Size .40 S&W a highly desirable addition.