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.223 REM 15 in. A1 1:12 No Type 2 Duckbill 605A: Model 605A: A1 Triangular S-1-F A1 Yes No .223 REM 15 in. A1 1:12 No Type 2 Duckbill 605B Model 605B: A1 Triangular S-F-1-3 A1 No No .223 REM 15 in. A1 1:12 No Type 2 Duckbill 606 CAR-15 Heavy Assault Rifle M1: A1 Triangular S-1-F A1 No No .223 REM 20 in. HBAR 1:12 Yes Type 2 Duckbill 606A
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.
If a 5.56×45mm NATO cartridge is loaded into a chamber intended to use .223 Remington, the bullet will be in contact with the rifling and the forcing cone is very tight. This generates a much higher pressure than .223 Remington chambers are designed for. [3] NATO chose a 178-mm (1-in-7) rifling twist rate for the 5.56×45mm NATO chambering.
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.
It also has the fastest rifling twist ratio for the .223 Remington of the three models(1:9 instead of 1:12). Early production rifles had a 1:12 twist rate and post-2014 rifles have the 1:9 twist rate. [2] The CZ 527 Varmint in .223 has muzzle velocities for standard-load 45-grain (2.9 g) ammunition of around 3,500 feet per second (1,100 m/s).
The .223 WSSM was introduced in 2003 by the Browning Arms Company, Winchester Ammunition, and Winchester Repeating Arms Company. The .223 designation is a reference to the popular .223 Remington. It is currently the fastest production .22 caliber round in the world with muzzle velocities as high as 4,600 feet per second (1,402 meters per second).
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.
Early models had barrels with a 1:12 rate of twist for the original .223 Remington, 55-grain (3.6 g) bullets. Current models have barrels with a 1:9 or 1:7 twist rate for the 5.56×45mm NATO, 62-grain (4.0 g) bullets.