A hard rest will cause the rifle to bounce in an unpredictable manner. A sticky rest or something that's not slick will do the same thing. I personally really like a bean bag full of beans. They form themselves to the rifle. They are slick you make them out of something like an old pair of jeans. Sand doesn't work as a filling because it's too hard. I've heard people say that Rice works. I suppose there are any number of things like that including packing peanuts which might work.