The .300 AAC Blackout (designated as the 300 BLK by the SAAMI and 300 ACC Blackout by the C.I.P.), also known as 7.62×35mm is a rifle cartridge developed in the United States by Advanced Armament Corporation (AAC) for use in the M4 carbine. Its purpose is to achieve ballistics similar to the 7.62×39mm Soviet cartridge in an AR-15 while using standard AR-15 magazines at their normal capacity. Care should be taken not to use 300 BLK ammunition in a rifle chambered for .223/5.56 or 7.62×40mm Wilson Tactical. While 5.56×45mm NATO has enjoyed widespread acceptance in military circles, the nature of the missions encountered by some special operations groups often demand a round that provides better performance than that available in the high-energy standard velocity rounds and subsonic performance greater than standard 9mm (the ubiquitous pistol round also commonly used in many SMGs).
Maximum estimated Combat effective range = *460 meters (503 yd) 125 grain supersonic. Maximum effective range tested = 800 meters. The Barnes 110 TAC-TX expands out to 300 meters from a 9" barrel and is a Law Enforcement and military round with barrier blind capability.
The 300 AAC Blackout was designed to achieve energies similar to the 7.62×39mm Soviet in an AR-15 while using standard AR magazines at their full capacity. The 7.62 Soviet's cartridge taper prevented reliable feeding in AR magazines and created wear on the bolt. From the 14.5 in (370 mm) barrel of the M4 carbine, the M855 5.56×45mm round has an effective point target range of 500 meters. The bullet has significant drop, drift, and energy loss at that distance. From a 16 in (410 mm) barrel, a 125 gr (8.1 g) .300 BLK round has a lower velocity and similar bullet drop and drift at shorter distances. However, it has the same amount of energy at 700 meters that the M855 has at 500 meters. In terms of hit probability, the Blackout has an effective range of 460 meters. From a 9 in (230 mm) barrel, the 125 gr BLK round has the same muzzle energy as the M855 from the M4, and 5 percent more energy at 440 meters. In comparison with 7.62×39mm rounds, .300 BLK rounds with varying loads have better ballistic coefficients and more energy out of similar length barrels. 300 BLK rounds like the Barnes TAC 110 grain, have "barrier blind" performance, being capable of penetration through several inches of different hard targets. 300 BLK allows a user to have one firearm with the capability of switching between subsonic, supersonic VMAX or barrier penetrating ammunition all with just the change of a magazine. It is able to replace the H&K MP5 for close quarters, and with just a magazine change, bring the fight to longer distances, outperforming the M4 carbine. The .30 caliber cartridge has an 89.1 percent increase in frontal bullet area over the 5.56×45mm, and so leaves a larger wound cavity in soft targets. It also penetrates deeper and initially yaws faster. 300 BLK rounds are effective out of barrels as short as 4.5 in (110 mm). Weapons chambered for the round can be as light, compact, and quiet when suppressed as submachine guns like the 9×19mm MP5, 5.7x28mm FN P90, and 4.6×30mm MP7 while having more energy and accuracy at longer range.