problem with rigid bodies and collisions

I am creating a logo wich involves a bullet that goes through a concreate box. I have followed a few tutorials and have setteld on using cell fracture to make the box into smaller pieces. Because  the box is shaped like a rectangel without a center, it is very instabel and collapses if i add an active rigid body to it. To fix this i putt the topp and bottom of the box to be animated instead of dynamic, but the part wich i want to be able to fracture still fracturs imedietly instead of when the bullet penetrates the box. I tried adding restraints but when I did the box stoped crumbeling, wich i good. but now when the bullet penetrates the box nothing happens! I tried deleting the constraints but now the bullet never collieds with the box. I dont now how to fix this and am wondering if anyone can help me?