The major difference in the different series of the rifles is as follows: The Magpul furniture gets better as you work your way up, The lower receiver gets better as you work your way up, and The handguard changes as you work your way up. For the Lower Receivers: Carbon Series gets our standard lower, Black Gold Series gets our new forging with the curved magwell cut, Diamond Series gets our new forging with the curved magwell cut plus lightning cuts in the trigger guard. For the Handguards: Carbon Series will get our standard handguards Black Gold Series will get our V-rails, Diamond Series will get our new anti-rotation S-rails.