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The 6.8 mm Remington Special Purpose Cartridge (aka 6.8 SPC, 6.8 SPC II & 6.8×43mm) is a rifle cartridge that was developed by Remington Arms in collaboration with members of the U.S. Army Marksmanship Unit, United States Special Operations Command〔(Not a private endeavor or fully sanctioned government project )〕 to possibly replace the 5.56 NATO cartridge in a Short Barreled Rifle(SBR)/Carbine. Based upon the .30 Remington cartridge, it is midway between the 5.56×45mm NATO and 7.62×51mm NATO in bore diameter and muzzle energy. It uses the same diameter bullet (not usually the same weight) as the venerable .270 Winchester hunting cartridge. == Development == The 6.8mm SPC cartridge was designed to address the deficiencies of the terminal performance of the 5.56×45mm NATO cartridge currently in service with the U.S. Armed Forces. The cartridge was the result of the Enhanced Rifle Cartridge program. The 6.8 SPC (6.8×43mm) was initially developed by MSG Steve Holland and Chris Murray, a United States Army Marksmanship Unit gunsmith,〔http://www.wilsoncombat.com/68project.htm〕 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 goal was to create a cartridge that would bridge the gap between 5.56 mm and 7.62×51mm NATO. The program started the design by 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. Armed Forces. In tests, it was 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. Further tests showed that a 6.8 mm caliber projectile was a compromise between the two, providing accuracy, reliability and terminal performance up to 500 meters. The combination of the cartridge case, powder load, and projectile easily outperformed the 7.62×39mm Soviet cartridge, with the new cartridge proving to be about faster. The resulting cartridge was named the 6.8 Remington Special Purpose Cartridge due to the size of its projectiles and the fact that it was based on the .30 Remington case. In general, adapting an AR-style rifle to the new cartridge only requires the replacement of the barrel, bolt, magazine & muzzle device (if applicable) of the 5.56 mm-chambered rifle; but to further streamline and simplify the conversion process many parts manufacturers sell complete upper receiver assemblies chambered for 6.8 SPC alongside their conversion kits focusing on the key individual parts. While a complete 6.8 SPC assembly is a somewhat more expensive route, the conversion of an existing 5.56 mm/.223 rifle to 6.8 SPC using a complete upper assembly takes less than a minute on an AR platform rifle without the need for specialized tools or skills. In contrast, when swapping out the individual component parts, a significant level of gunsmithing experience, special tools, and time are generally required to detach the barrel from the rifle's upper receiver and the gas system, and conversely those same needs are required for the reassembly of the upper receiver with the new 6.8 SPC barrel. Also, there is the issue of having to readjust the sights if a new barrel is placed on an existing upper receiver. The 6.8 mm Remington SPC was designed to perform better in short barreled CQB rifles after diminished performance from the 5.56 NATO when the M16A2 was changed from the rifle configuration to the current M4 carbine. The 6.8 SPC delivers 44% more energy than the 5.56 mm NATO (M4 configuration) at . The 6.8 mm 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 lighter, allowing operators to carry more ammunition than would otherwise be possible with the larger caliber round. The 6.8 mm generates around of muzzle energy with a bullet. In comparison, the 5.56×45mm round (which the 6.8 is designed to replace) generates around with a bullet, giving the 6.8 mm a terminal ballistic advantage over the 5.56 mm of . One of the enigmatic features of this cartridge is it being designed for a short barrel carbine length rifle that the standard rifle length is (usually ). The round only gains about for every 25 mm of barrel length past the standard barrel (all else being equal) up to barrel's length around with no gain/loss in accuracy. It also does very well in rifles with less than barrels. In recent developments (the period 2008-2012) the performance of the 6.8 SPC has been increased by approximately by the work of one ammunition manufacturer Silver State Armory LLC (SSA) and a few custom rifle builders using/designing the correct chamber and barrel specifications. Also, more recently, LWRC, Magpul and Alliant Techsystems (ATK) are currently introducing a new AR platform designed for the 6.8 SPC which allows for a proprietary 6.8 Magpul P-Mags and an overall cartridge length of . The Personal Defense Weapon (PDW) known as the Six8 is SPC II w 1: twist and is able to use all current 6.8 SPC factory ammunition.〔https://www.lwrci.com/articles/SGNLWRCsix8.pdf〕 See Gold Dot below for ATK's part. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「6.8mm Remington SPC」の詳細全文を読む スポンサード リンク
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