Well, to be fair, I did spell it incorrectly. It's a rail "bungee." As to what it is or what practical purpose it serves, I haven't the foggiest idea. I saw one on a guy's rifle at a class awhile back and asked him why he'd tied his anorak cord onto his rifle. He scoffed and told me what it was. Apparently a company called "Warsport" makes them.Whut are rail bungies? I'm old fashioned. I do know (I think) what a 'rail' is.
http://shop.warsport.com/war-sport-rail-bungee/