Originally Posted by stAtrill
If you have a background thats a solid color and a foreground thats an obstacle, then you can put a hole in it.
What you would do, is draw a sprite the size of the hole you want and the same color as your background. Then, when your bullet hits the wall, create the active there, paste it into the background (NOT as an obstacle), then destroy the sprite, all in one line, and in that order. To change the order (say you messed up), then just double click the line to enter the order editor.
Hope i covered everything. :)