Steel plates loosely mounted in frame, with neoprene pads between the frame and plates to better absorb the noise and impact from projectiles. The color signature may be matched with the other signatures in an attempt to identify the object. Each of the engraved or embossed marks is preferably painted with a distinguishable color along a circumferential surface. The blocks are made by placing used tire stacks in a form that centralizes the stacks so that reinforced concrete can be placed around them while maintaining a hollow core through the stacks. After detecting a first feature having at least one end thereof to be in the proximity of one end of a second feature, a first OPC pattern is incorporated to the end of the first feature toward a first direction; and a second OPC pattern is incorporated to the end of the second feature toward a second direction that is substantially opposite from the first direction.