Recommend starting with a standard carbine buffer (and it's the cheapest). If your BCG is going back too fast, too soon, and the extractor is ripping off the rim of the case, then go to a heavier buffer to slow down the BCG and give the chamber pressure more time to go down before extraction. And be sure to install an O-ring over the extractor spring. A #60 will do. Buy a 10 pack of them at Lowes in the plumbing supply area for around $2.00.