The barrel nut/jam nit design is well known and massively outdated. We had the same set up on our Bushmasters overseas. The nut is to be tightened and aligned with the receiver. The rail slips over the nut and is tightened down/rotated until is almost to the rear. You then rotate the jam nut towards the rail and torque it down. Once that is done you then tighten the anti-rotation screws.
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