AR-15 5.56mm and .300 BLK Ammunition Compared

A few years ago, a company called Advanced Armament Corporation came up with the brilliant idea to create an .300 caliber AR-15 system that could use existing lower receivers and existing ammunition magazines, requiring only the quick swap of one upper receiver for another. The result is a rifle for the AR-15 base system that employs ammunition that is legal for hunting. To an extent, some branches and departments of the U.S. military, have been experimenting with .300 BLK/Blackout as a way to easily increase the caliber of ammunition used in its otherwise standard issue array of carbines. 

Three .300 BLK variants, 5.56mm x 45mm NATO, and 7.62mm x 39mm

Below is a visual comparison between 5.56mm x 45mm NATO ammunition and .300 Blackout aka 7.62mm x 35mm ammunition. Both types of ammunition fit into standard AR-15 magazines, and both maintain the same magazine load capacities. From left to right in the photograph below are: 300 AAC Blackout plastic tipped, .300 AAC BLACKOUT 125 match, .300 AAC BLACKOUT 220 subsonic, 5.56mm, and 7.62×39mm ammuntion.

Part of what makes the .300 BLK platform convenient is that anyone with an existing AR-15 rifle can purchase a complete .300 BLK upper receiver and snap it right onto the lower receiver of their existing rifle and essentially have two rifles in one. Since most states hunting statutes require a minimum caliber of .243, the .300 BLK caliber is adequate for hunting, so long as other regulations and restrictions are adhered to. For example some jurisdictions limit the number of rounds that can be loaded in a magazine at any given time. The .300 BLK camber caliber can also be useful to developing marksmanship at longer ranges, since the high calilber round has improved long range capabilities. Companies such as Nikon have developed scopes specifically for the .300 BLK platform as well.