300 AAC Blackout 300 AAC Blackout, SAAMI short name 300 BLK, also known as 7.62x35mm is a rifle cartridge developed in the United States and originally chambered in the M4 carbine. Since its inception as the standard US military primary issue rifle ammunition, the 5.56×45 mm NATO round has run up against firm opposition from those who argue ... May 04, 2016 · .223, .300 BLK and .308 Winchester – the .300 BLK is a unique and specialized cartridge. The .300 Blackout is a popular option for many reasons. First, the conversion works and is reliable. The bugs are worked out, and the rifle will feed function and run with .300 BLK ammunition. Jan 29, 2019 · 300 Blackout vs 223. The cases used for the 300 BLK can be potentially created by taking standard 223 cases, and manipulating them to fit the 300 BLK system. The only thing you need to change in order to convert your 5.56 M16 rifle to a 300 BLK M16 rifle is the barrel, and possibly the muzzle break. Otherwise, they are quite similar. 300 Blackout ammo is legal for hunting deer in almost every state as long as you use supersonic ammo, and it's legal in many states with subsonic ammo, too. Some people question whether or not the 300 Blackout is effective for killing deer, and the simple answer is, yes—absolutely.