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Some of the carbon fiber uppers use a proprietary barrel nut but other than that I think the rest is interchangeable. I've got a 7inch upper (not on a carbon fiber upper) with the gas port moved back 1.5 inches from pistol position. It will run subsonic loads down to 110 grn bullets no problem. I have two carbon fiber uppers and lowers running other things. One thing I have noticed on the carbon fiber uppers is that they do flex more than other uppers. You can really see this when tightening up the barrel nut.
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