What are you talking about?
@EugenFJR is referring to the NM hand guard like I have on my CMP rifle. You use hallowed out A2 handgaurds around the free float tube and the sling swivel is not attached to the FSB, it's on the free float tube. You can crank down on the sling without causing barrel deflection. Would be a solution to your worry about POI change while using a sling. Great option if you want to keep the A2 look.
Seems rude the way you responded to a good suggestion.
Down side is weight. That RR setup is HEAVY, so much in fact, that you need to add lead to the A2 stock cavity if you want to balance the rifle back out.
Below: RR NM free float setup vs a standard A2 FSB/end cap
Standard A2 FSB with end cap
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RR NM free float setup (note you either need to use a pre-ground off sling swivel FSB, or file yours off like mine is an cold blue it) if you want the A2 FSB that is...
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Note how similar it looks to a tradition A2 upper
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