I have been wondering the same thing on a 11.5" barrel with a 5" flash, so I have no idea why they do that, A 14.5" barrel with a flash makes a little more sense if they want to keep the overall length down. I assume you are talking about rifle length and carbine length buffer springs and tubes, they are not interchangeable, short spring goes with short tube and collapsable stock. A carbine length gas system is apx 7" long, the rear of the gas block will be 7"+- down the barrel, A mid gas will be at 9"+-, A mid length gas system will delay the opening of the bolt and the cycling of the action, they can be a little easier on the system(bolt and cam pin mainly). The piston system has a piston in a cylinder around the gas block location(every system is different) and a rod replaces the gas tube.
The 6.8 is the fastest growing cartridge and #1 choice for hunting deer and hogs with an AR15.
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