The cotton M3 bandoleer had seven pockets that were each designed to contain a cardboard sleeve that held a 10-round clip of 5.56mm ammunition for the M16 rifle. In Vietnam troops would often carry a 20-round M16 magazine in each of the bandoleer's seven pockets and would frequently strap two, and sometimes even three, bandoleers across their chests.
Not only were the bandoleers lighter than the standard issue magazine pouches, they were also less bulky and allowed faster rifle reloading. Though limited testing in 1968 proved that nylon bandoleers were more durable and lighter when wet than the standard cotton version, they were never officially adopted.1
1. Jungle Snafus...and Remedies by Cresson H. Kearny (Oregon Institute Of Science And Medicine 1996)