Well, the grip screws onto the classic but it seems to have slightly larger gaps all around. The main challenge is that on the classic, unlike the standard 556, the grip holds the spring that works with the retained rear pin. The CAA grip wall is thinner, and once modded to fit a 556 standard, a gap exists where it smushes down onto that spring. The spring is depressed by the grip with reasonable pressure but you see that it isn't depressed all the way flush with the surface of the receiver and the spring remains exposed from the side in that gap. A dab of JB Weld or some other material might be needed to make this modded grip work on the classic.