300AAC Blackout and 6.8SPC

A fairly interesting question was posed to me by a co worker yesterday afternoon. He wanted to know which round is bigger, 6.8SPC or the .300AAC Blackout? I had to say, I honestly didn’t know, since both rounds had been developed and came to the public view after I had retired from the military and I had no knowledge of them in my military career. Having worked as a firearms instructor, I should have know, but I didn’t.

So I decided it was time to find out.

The .300 AAC Blackout round is an AR platform round, developed to be used with silencer equipped firearms. The size of the .300 is 7.62x35mm. Anyone who has fired a .308 knows what this means. The round is measured by the diameter of the barrel of the firearm it is intended to be fired from. A barrel is typically measured by the distance between grooves in the barrel, although it can be measured as the distance between lands as well. When the distance is measured in inches, we typically will hear the term “cal” being used to describe this measurement, as in .308 cal. When the distance is measured using metrics, we will typically hear “7.62x39mm” or “7.62x54mm”. The number expressed in mm’s is the length of the cartridge, while the decimal number is the distance between the grooves in the barrel. So a 7.62X39mm is shorter than a 7.62×54, but the diameter of the round is the same. NOTE: BE VERY CAREFUL FIRING .308 THROUGH A GUN RATED FOR 7.62X51. WHILE THE CARTRIDGES MAY LOOK IDENTICAL, THEY ARE NOT. THE CHAMBER PRESSURES ARE DIFFERENT FOR THESE FIREARMS AND USING THE WRONG AMMO CAN RESULT IN DAMAGE TO YOUR GUN. GENERALLY, YOU CAN FIRE 7.62X51 IN A .308 SAFELY.  CONSULT YOUR OWNER’S MANUAL OR A GUNSMITH IF YOU HAVE DOUBTS.

So now that we understand how rounds are measured, let’s compare rounds. The .300AAC Blackout round, as mentioned previously is a 7.62x35mm. The 6.8SPC is measured as 6.8x43mm. So the answer to the question is clearly that the 6.8SPC is longer than the .300AAC. However, the .300AAC is wider than the 6.8SPC.

So now that we have come to understand the difference between the two rounds, let’s discuss why they were developed in the first place.


The .300AAC Blackout was developed by Advanced Armament Corporation to meet the need for a round in the .30 cal range to be used with suppressed firearms and and have the same energy properties as the 7.62 Soviet rounds. The .556 has been found to be lacking in stopping power, so AAC began development of new round that had the increased power at a shorter distance equivalent to the 7.62 rounds. The unique thing about the .300AAC is that it can be used with a simple barrel change, as opposed to the 6.8SPC, which requires a barrel, bolt and magazine change in order to be used.

Remington Arms developed the 6.8SPC in response to the need for a longer range round for the AR platform. The new round developed outperformed the Soviet 7.62 round. The need for a round that would perform well in the current short barreled M4 platform had become apparent to the military. The 6.8SPC does this well, but a complete upper conversion is necessary in order to make it work.

AR Platform Rounds



The work to find a new AR platform round that would meet the needs of today’s military has generated both of these rounds, and while they are both unique in their own way, they are definitely different. The question remains which one will eventually come out on top. The unique characteristics of firearms today mean that both rounds are available to anyone who has the time or the passion to convert their existing AR platforms for whichever round they prefer.


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