It also does well in rifles with less than 410 mm (16 in) barrels. The round only gains about 7.6–10.7 m/s (25–35 ft/s) for every 25 mm of barrel length past the standard 410 mm (16 in) barrel (all else being equal) up to barrel's length around 560–610 mm (22–24 in) with no gain or loss in accuracy. One of the enigmatic features of this cartridge is its being designed for a short-barrel carbine length rifle than the standard rifle length is (usually 41 cm (16 in)). In comparison, the 5.56×45mm round (which the 6.8 is designed to replace) generates around 1,796 J (1,325 ft⋅lbf) with a 4.0 g (62 gr) bullet, giving the 6.8 mm a terminal ballistic advantage over the 5.56 mm of 588 J (434 ft⋅lbf). The 6.8 mm generates around 2,385 J (1,759 ft⋅lbf) of muzzle energy with a 7.5-gram (115 gr) bullet. The 6.8mm SPC is not the ballistic equal of the 7.62×51mm NATO cartridge, but it has less recoil, has been said to be more controllable in rapid fire, and is lighter, allowing operators to carry more ammunition than would otherwise be possible with the larger caliber round. The 6.8mm Remington SPC was designed to perform better in short-barreled CQB rifles after diminished performance from the 5.56 NATO when the M16A4 was changed from the rifle configuration to the current M4 carbine. The combination of the cartridge case, powder load, and projectile easily outperformed the 7.62×39mm and 5.45×39mm Soviet cartridges, with the new cartridge's muzzle velocity proving to be about 61 m/s (200 ft/s) faster than the 7.62 x 39. 30 Remington parent case, Holland and Murray determined that a 6.5 mm caliber projectile had the best accuracy and penetration, with historical data going back for decades of US Army exterior and terminal ballistic testing, but a 7 mm projectile had the best terminal performance. In tests comparing various caliber bullets using a. 30 Remington case, which was modified in length to fit into magazines that would be accommodated by the magazine wells of the M16 family of rifles and carbines that are currently in service with the U.S. The program started the design by using a. The 6.8 SPC (6.8×43mm) was initially developed by Master Sergeant Steve Holland and Chris Murray, a United States Army Marksmanship Unit gunsmith, to offer superior downrange lethality over the 5.56 NATO/.223 Remington in an M16-pattern service rifle with minimal loss of magazine capacity and a negligible increase in recoil. The cartridge was the result of the Enhanced Rifle Cartridge Program. The 6.8mm SPC cartridge was designed to address the deficiencies of the terminal ballistics of the 5.56×45mm NATO cartridge currently in service with the armed forces of all NATO-aligned countries. It uses the same diameter bullet (usually not the same mass) as the. 30 Remington cartridge, it is midway between the 5.56×45mm NATO and 7.62×51mm NATO in bore diameter. Army Marksmanship Unit and United States Special Operations Command to possibly replace the 5.56 NATO cartridge in short barreled rifles (SBR) and carbines. The 6.8mm Remington Special Purpose Cartridge ( 6.8 SPC, 6.8 SPC II or 6.8×43mm) is a rimless bottlenecked intermediate rifle cartridge that was developed by Remington Arms in collaboration with members of the U.S. 5.5 g (85 gr) Barnes TSX with "tactical" factory SSA load
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