The problem you are going to have is the "explosive" effect which you seem to be seeking. Bullets will tend to punch through a plaster block rather than creating what I feel you are looking for. They may crack up quite a bit but I am thinking you are looking more for that shattering effect rather than a punch through. What you COULD do it find a tannerite target http://www.tannerite.com/ or a small jar of it and load the plaster block with a small charge of that and THEN shoot it. You will get a nice explosive breakage for filming in slow motion.