Ice will shatter like glass if enough energy is put in, crater on the surface if it isn't and may allow small diameter projectiles with enough energy to punch clean through without shattering the whole.
Much less awkward to shoot than actually getting a slab of glass 6" thick
Everyone likes seeing sparkly pieces flying about ^_^
Just require a sizeable freezer, a large tub - perhaps a deep plastic biscuit box, fill with water, possibly food colouring and leave for 3 days to a week.
You may need to flatten off the top because it has a tendency to freeze at the edge then the middle, causing the middle to erupt upwards. A saucepan filled with hot water makes a good ice flattening tool.
I made one block some time ago but didn't find time to shoot it.
