The AK-74 was designed by А. D. Kryakushin's group under the design supervision of Mikhail Kalashnikov. It is an adaptation of the 7.62×39mm AKM assault rifle and features several important design improvements. These improvements were primarily the result of converting the rifle to the intermediate-calibre high velocity 5.45×39mm cartridge. In fact, some early models are reported to have been converted AKMs, re-barreled to 5.45×39mm. Compared with the preceding AKM, the AK-74 has better effective firing range, accuracy (a main development goal), and reliability. About 50% of the parts in the AK-74 and AKM are the same (including pins, springs and screws). he rifle's operation during firing and reloading is identical to that of the AKM. After ignition of the cartridge primer and propellant, rapidly expanding propellant gases are diverted into the gas cylinder above the barrel through a vent near the muzzle. The build-up of gases inside the gas cylinder drives the long-stroke piston and bolt carrier rearward and a cam guide machined into the underside of the bolt carrier along with an ejector spur on the bolt carrier rail guide, rotates the bolt approximately 35° and unlocks it from the barrel extension via a camming pin on the bolt. The moving assembly has about 5.5 mm (0.2 in) of free travel which creates a delay between the initial recoil impulse of the piston and the bolt unlocking sequence, allowing gas pressures to drop to a safe level before the seal between the chamber and the bolt is broken. Like previous Kalashnikov-pattern rifles, the AK-74 does not have a gas valve; excess gases are ventilated through a series of radial ports in the gas cylinder. Since the Kalashnikov operating system offers no primary extraction upon bolt rotation, the 5.45×39mm AK-74 bolt has a larger extractor claw than the 7.62×39mm AKM for increased extraction reliability.[19] Other minor modifications were made to the bolt and carrier assembly. The rifle received a new barrel with a chrome-lined bore and 4 right-hand grooves at a 200 mm (1:7.87 in) or 37 calibers rifling twist rate. The front sight base and gas block were redesigned. The gas block contains a gas channel that is installed at a 90° angle in relation to the bore axis to reduce bullet shear at the port hole. A pair of support brackets are cast into the gas block assembly and are used to attach a BG-15c or GP-25 underslung 40 mm grenade launcher. Like the AK-47 and AKM, the muzzle is threaded for the installation of various muzzle devices such as the standard muzzle brake or a blank-firing adaptor, while a spring-loaded detent pin held in the front sight post prevents them from unscrewing while firing. However the muzzle threads have been relocated to the front sight base for both easier and more economic replacement in case of thread damage. The distinctive standard-issue muzzle brake features a large expansion chamber, two symmetrical vertical cuts at the forward end of the brake and three non symmetrical positioned vent holes to counteract muzzle rise and climb as well as lateral shift to the right much like the AKM's offset muzzle brake. A flat plate near the end of the brake produces a forward thrust when emerging exhaust gases strike its surface, greatly reducing recoil. The muzzle brake prevents backblast from reaching the firer, although it is reported to be harsh on bystanders as the muzzle gases are dispersed to the sides.[20] The standard-issue AK-74 muzzle brake has been subtly revised several times since the 1970s.