A firearm assembly including a receiver, a barrel, a locking block and a slide configured to provide rapid changing of a barrel of a handgun wherein the barrel may be for an alternate caliber round of ammunition. The locking block is configured to be secured within the receiver and includes an upper surface that has front rise member, a rear rise member and a valley intermediate thereto. The upper surface of the locking block is mateably shaped with the second portion of the barrel, in particular the lower portion thereof. The lower portion includes a front shoulder member, a rear shoulder member and a notch formed therebetween wherein the notch is configured to mateably engage the rear rise member of the locking block. The slide of the present invention includes a void intermediate the sidewall and the extractor housing. A shim is provided to be secured within the void.