300 Blackout Upper

The Best 300 Blackout Upper 

For the M4 carbine, Advanced Armament Corporation (AAC) of the United States created the .300 AAC Blackout (classified as the 300 BLK by the SAAMI and 300 AAC Blackout by the C.I.P. While the military has widely adopted the 5.56x45mm NATO, some special operations organizations require a cartridge with stronger subsonic performance than the regular 9x19mm Parabellum (a common pistol and submachine gun round) because of the nature of the missions they face. 

This need prompted AAC, in conjunction with Remington Defense, to create the .300 AAC Blackout. Many of the perceived limitations of other large-caliber cartridges used in the M4 were hoped to be mitigated by the new cartridge. There were issues when Colt Firearms and others tried to chamber AR-style rifles and carbines for use with different .30 caliber ammunition. The 7.62x39mm was notoriously difficult to feed properly with standard AK-47 magazines unless modified, and even then the results were less than ideal. Because of the increased size of the case head, special bolts were also required. Both the 6.8 SPC and the 6.5 Grendel had problems with interchangeable components, but they could use the 30-round magazines designed for the M4 and M16.

Although some wildcat cartridges, such as the .300 Whisper series, have solved these problems, their lack of an industry-standard cartridge diameter and widespread use in single-shot pistols have rendered many popular loads, both supersonic and subsonic, subpar in AR-type firearms. However, several of these cartridges had powder charges that were incompatible with the M4 carbine's requisite pressure, and their overall cartridge length was too great to allow loading in a STANAG magazine. The combination of subsonic ammo with a suppressor led to short stroking and heavy fouling, much as it had in Vietnam with the original M16 models.

By focusing on the M4 and M16 as major platforms, designers were able to create a wide variety of cartridges that not only met the ballistic standards specified but also ensured mechanical reliability with the minimum number of modifications to the weapon itself—just a barrel swap.

300 BLK Versus the .223/5.56 NATO Round 

The 300 BLK's strength lies in its compatibility with existing ammunition designed for the more common .223/5.56 caliber, but this can also pose a safety risk if the two are mixed. SAAMI has blacklisted using a 300 BLK round in a .223 chamber due to the comparable chamber dimensions of the two calibers. Chambering and firing a 300 BLK bullet in a .223 rifle can cause the gun to explode, putting the shooter in danger. This is because the 300 BLK bullet is larger than the .223 bore

Some have suggested that owners of firearms in both calibers take extra precautions to prevent accidental mixing of ammunition by correctly identifying firearms and magazines and physically inspecting each round before loading. The bullet length, bullet profile, bullet seating depth, crimping, and powder load all play a role in determining whether or not a 300 BLK cartridge will chamber in a .223 barrel. A perfect cartridge would have a longer projectile, a powder charge that completely filled the case, and the projectile crimped into place.

Shop now at Gorilla Machining, we carry all of the necessary accessories and parts to build out your dream 300 AAC Blackout Rifle or Pistol!

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