Im looking to build my first AR and just began looking at the different uppers. I think I'm leaning a little toads the CMMG 16" line but when I go to their website I get a little overwhelmed. For example this upper CMMG 16" M4 Carbine Upper, www.cmmginc.comCMMG Inc. AR15 Rifles, Parts and Accessories. .22 LR Dedicated AR-15 Rifles, Dedicated AR 15 9mm Rifles and Uppers, Tactical Bacon has a ton of additional options for "gas piston system" etc. With the original price is there already a gas system in there and these are just upgrades or do I have to pick one of each in all the options? Any help would be appreciated. Thanks guys!