I made an AR10 version of my Eject-able 223/AR15 chamber flag.  I used a resized/deprimed 45 ACP brass cut down to 15mm or so as the case head. (308 and 45 ACP have the same head size and I have an abundance of 45 brass)  The brass opening in the model may be a snug fit, especially if you do not use resized brass or elect to use rifle brass.