The 300 AAC Blackout(7.62 × 35 mm), SAAMI abbreviation "300 BLK," is a rifle cartridge developed in the United States and initially housed in the M4 rifle.
The 5.56 × 45mm NATO round since its inception as standard US military primary issue rifle ammunition has gone against the firm opposition of those who argue that a 30 caliber round is the minimum, if not the ideal, modern battlefield. Provides the necessary performance for the soldier. While the 5.56 mm NATO has gained widespread acceptance in military circles, the fluid nature of the mission that faces certain special operations groups often calls for around that not only outperforms in high-energy standard velocity rounds  Provides but may also offer a subsonic performance higher than the current standard 9mm submachine gun that is now in use. 
300 AAC blackout to fulfill this requirement (whose existence was first made public by an article in the Military Times.)  Created by Advanced Armament Corp. in collaboration with Remington Defense, directing AAC's research and development Was. Director Robert Silver.
The goals of the project were:
Create a reliable compact 30-cal solution for the AR platform
Use existing inventory magazine while maintaining its full potential
Create the optimal platform for sound and flash suppressed fires
Create compatible supersonic ammo that matches 7.62 × 39mm ballistic
Provide the ability to penetrate obstacles with high-mass projectiles
Provide all the capabilities in a lightweight, durable, low recoiling package
They were meeting these goals allowed the development team to deny many of the perceived deficiencies inherent in other large-caliber cartridges when used in the M4 platform. Colt firearms and other weapons manufacturers previously housed AR-pattern rifles and carbines in various 30 caliber rounds but faced several issues. In the case of 4.72 × 39, its relatively severe case angle resulted in feeding problems  until specially modified AK47 magazines were used, and the results were still less than outstanding. Modified bolts were also required due to their larger case head diameter. Rounds such as 6.8spc and 6.5 Grendel had similar part-interchangeability issues but allowed standard M4 / M16 30 round magazines with reduced capacity.
Wildcats such as the 300 Whisper and 300–221 addressed these issues. Still, their widespread use in single-shot handguns with a lack of industry-standard cartridge dimensions meant that a large number of popular loads on both the supersonic and subsonic end of the spectrum. AR patterns were less than ideal in weapons. Many of these rounds required a highly long composite cartridge length that prohibited feedings to a STANAG magazine using powder charges that were not compatible with the pressure requirements of the M4 carbine. This was particularly noticeable when subsonic ammunition was used in combination with silencers because short stroking and excessive fouling would be similar to what was seen in early versions of the M16 in Vietnam. 
Considering the M4 as the primary host during load development, designers were able to design multiple cartridges that would meet the prescribed ballistic requirements and ensure mechanical reliability with the lowest changes to the weapon with only one. A simple barrel change is required for complete conversion.
Sami approved the 300 AAC blackout on January 17, 2011.
The component primed brass was delivered in March 2011.
On October 23, 2011, USAMU's SSG Daniel Horner used a 300 AAC blackout to win his fourth USPSA Multi-Gun National Championship.