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 its being designed for a short-barrel carbine length rifle than 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 or loss in accuracy. It also does 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 ammunition manufacturer Silver State Armory LLC (SSA) and a few custom rifle builders using and designing the correct chamber and barrel specifications. The 6.8mm Remington SPC cartridge weighs, depending on the manufacturer and load, between . Also, more recently, LWRC, Magpul and Alliant Techsystems (ATK) introduced a new AR-15 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. Muzzle velocity from a barrel Muzzle velocity from a barrel Muzzle velocity from a barrel Comparison to other military calibers Typical trajectory information from carbines with drop and velocity calculated at sea level with a zero. Applications Military and law enforcement adoption By late 2004 the 6.8×43mm SPC was said to be performing well in the field against enemy combatants in special operations. However the cartridge was not used by conventional US military personnel. It was not adopted for widespread use due to resistance from officials. The 6.8 SPC was designed for better terminal effectiveness at the shorter ranges of urban combat experienced in Iraq. When fighting in Afghanistan began to intensify, engagements began taking place at greater distances, where the 6.8 SPC begins to falter. Experiments suggested that the comparatively short 6.8 mm bullets became ineffective at longer ranges. In 2007, both the U.S. SOCOM and the U.S. Marine Corps decided not to field weapons chambered in 6.8 mm due to