The "plastic pistol" is popular because of its lightweight, ergonomics, environmental resistance, magazine capacity, and ease of operation. The XD-45ACP is built on Springfield Arms' XD model, which locks by means of the barrel hood moving up into and bearing on the front edge of the ejection port. When the pistol is fired, the barrel and slide move together a short distance before the barrel drops down on a frame-mounted locking block. This permits the slide to continue toward the rear, extracting and ejecting the spent cartridge case. As the recoil spring pulls the slide forward, it strips the next round out of the magazine and chambers it. When the slide goes into battery, the barrel is cammed up, whereupon its hood enters the ejection port and locks the two units together. The XD uses what Springfield refers to as its Ultra Safe Assurance (USA) trigger system. This requires a long but light trigger stroke that overcomes the trigger and firing pin safeties before it trips the sear and releases the striker to fire the pistol. Until recently, Springfield offered the XD chambered for the 9mm Parabellum, .357 SIG, .40 S&W, and .45 GAP. In order to offer the XD chambered for the .45 ACP, it was redesigned and upsized to the XD-45ACP, models are 5 and 3 ounces heavier than the XD 9mm and .40 caliber model, and a quarter of an inch longer. The staged column, steel magazines hold 13 rounds of .45 ACP ammunition. Performance testing has been impressive. Steps for its disassembly are listed.