6.5x40 - a 6.5 SPC Improved

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The AR-15 platform is not just limited by it's length, It is also limited by it's case head/rim diameter. The POF Revolution uses an AR-10 bolt and barrel extension(modified), with a modified BCG. By upping the parent cartridge to a .308, shortening 39/38mm you have a fatter Grendel which should provide more strength to the bolt lugs and thus allow you to take advantage of the extra powder capacity and perhaps even run it a little hotter.