My first AR was the Ruger AR556, which has a proprietary D-Ring which screws on/off. My 2nd AR is a PSA which has the normal D-Ring. If the Ruger D-Ring is tightened too much it takes 2-1/2 men to loosen the thing. I've actually used a rubber strap to get mine off. The newer Rugers have a M-Lok foregrip, did they keep their D-Ring, use the normal D-Ring, or does the M-Lok grip do away with the D-Ring? If the M-Lok grip does away with it,what holds it in place and can my Ruger AR be retrofitted?