Make a line where you want to cut it using a hammer and a wide mason's chisel. Just light taps enough to make the line. Continue back and forth along the line. Do the same on the bottom in the same position as the top line. Alternate back and forth slowly, very slowly, increasing the hammer blow force as you go, and eventually the block will crack open at about the point that you want it, if you don't hit it too hard. This works better with practice and doesn't always give you a nice straight cut. Or you could get a concrete abrasive blade and use it in your oldest circular saw, cutting a slice in from opposite sides. Once you are about 1/3 of the way through from each side, a slight blow of a hammer will usually break it the rest of the way. If successful, repeat the process on the other end of the block. Concrete never breaks with nice clean lines. There is too much variation in the bonds between the concrete and aggregate inside them. The light blows with the chisel sets up a crack line through the block to the crack line coming from the other side. The saw cut does similar, if you have an old spare circular saw. (wouldn't use a nice one for this).
A right angle grinder with a masonry disk can clean up the break.
Of course, you want to use a good dust mask, safety glasses, a face shield it a good idea too, as well as leather gloves.
Charley
Oh yeah, check with BATF first. They're pretty sensitive about that stuff. 
