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Expert Answer
The Luth-AR 9mm Carbine Buffer Assembly would work but you may experience feeding/ejection reliability issues. Buffer weight for the AR 40 is going to be a bit lighter than that range (5.4-7.4 oz for AR9) due to the larger caliber having more pressure behind the round when fired. I would suggest buying a heavy tungsten buffer weighted at 4.3 oz paired with a pistol/carbine buffer spring. Email us at Techsales@opticsplanet.com if you have any further questions.