The full auto bolt has a lip fully around the bolt face and especially on the bottom. The semi auto bolt lacks the lip below the firing pin. This prevents it from chambering and firing the next round if the striker cycles with the bolt. The semi auto extractor is suppose to be smaller for the same reason but the Chinese Uzi has a full auto extractor. The later semi auto's also have a cam on the back of the bolt that slows down the strike if it follows the bolt and it also prevents out of battery firing. It ams on the back of the late short semi auto ejector. The full auto ejector is taller I believe as is the early semi auto.