There is a million things you can pack you bags with, lead shot, corn cob, rice, and many other things, but sand is what is most common. I guess that is why they are nicknamed "sand bags." Even with sand there is different types of sand, from play ground variety, to the heavy sands, and even some things that are called sand, and they look like sand but they are not sand, an example is coal slag. Some people even mix sands to get a top notch mix. Dave Dorman (RIP) use to sell super fabric bags made off of protektor bags, and I was told he used a mixture of coal slag, and zircon.

When I buy my bags I buy them pre-filled, heck it cost a bit more in shipping, but I cant buy heavy sand, and fill them myself cheaper. I bought a bag of coal slag last year (Available at any true value.) http://www.idealtruevalue.com/servlet/the-80507/Detail cost about $10 bucks for 50 lbs, and this stuff is heavy. I think it is just as heavy as zircon sand. I have also been told that most Zircon sand is not truly zircon.

Getting back your your question. I find that on a front bag if you fill them too full the rifle will bounce in the rest, you need to leave some wiggle room. A bit of information I was handed down from a few shooters is you want to have sand between the gun and the rest because it will gobble up mistakes. On the rear bag, I fill them as full as I can get them. Last your bunny ear bag is a number 14.