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The rifling twist rate for this cartridge is 177.8 mm (1 in 7 in), 6 grooves right hand twist, Ø lands = 5.56 millimetres (0.219 in), Ø grooves = 5.69 millimetres (0.224 in). [6] According to STANAG 4172 and the official NATO proofing guidelines the 5.56×45mm NATO case can handle up to 420.0 MPa (60,916 psi) piezo service pressure.
NATO chose a 178-mm (1-in-7) rifling twist rate for the 5.56×45mm NATO chambering. The SS109/M855 5.56×45mm NATO ball cartridge requires a 228 mm (1-in-9) twist rate, while adequately stabilizing the longer NATO L110/M856 5.56×45mm NATO tracer projectile requires an even faster 178 mm (1-in-7) twist rate.
The 457 mm-long barrel, has six right-hand grooves and a 228 mm (1:9 in) twist rate. The barrel's external profile (from the breech to the front sight base) is identical to the Tantal's barrel, but the length from the foresight base has a visible tapered contour with a decreasing diameter towards the muzzle end, used to mount a flash suppressor.
Weapons designed to adequately stabilize both the M193 or SS109 projectiles (like civilian market clones) usually have a six-groove, right-hand twist, one turn in 9 inches (1:228.6 mm or 41.1 calibers) or one turn in 8 inches (1:203.2 mm or 36.5 calibers) bore, although other and 1:7 inches twist rates are available as well.
The barrel has a chrome-lined bore with 6 right-hand grooves and a 178 mm (1:7 in) rifling twist rate that is optimized for use with heavier SS109 5.56×45mm NATO rounds. A barrel with a 305 mm (1:12 in) twist rate designed specifically to stabilize the lightweight M193 cartridge is also available. [ 2 ]
The rifle's barrel features 6 right-hand grooves and a rifling twist rate of 1 in 178 mm (1:7 in); it is fitted with an open-type 3-prong flash suppressor, and later a "birdcage" style one, both designed to launch rifle grenades. The weapon will also mount a knife-type bayonet.
By intentionally choosing a slow twist rate the 55 grain bullet used in the 5.56 Ball M193 cartridge was only just stable in flight. The damage caused by the 5.56 mm bullet was originally believed to be caused by instantaneous "tumbling" on impact and render a wide, incapacitating wound due to the slow 1 in 14-inch (360 mm) rifling twist rate.
The K2 uses the same magazine as the M16. The barrel rifling has 6 grooves, 185 mm (1-in-7.3) right hand twist. The K2 has 3 selective firing modes: semi-auto, 3-round burst, and full automatic. KNP combat police officers armed with Daewoo K2s. Note the folded buttstock.