For .300 Blackout AR15 carbines, they tend to run softer than 5.56/.223 AR15s, especially if you use subsonic ammunition.  So in order to cycle reliably and not short-stroke, we recommend using the RB5000L (Light Model) and the Sprinco Yellow Reduced Power Buffer Spring.  The RB5000L is 4.0 oz. instead of the 4.2 oz. of the standard RB5000 model buffer and the buffer has a reduced power piston return spring and different hydraulics.  Combining this buffer with the reduced power buffer spring has been a good combination for .300 Blackout Shooters.  

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