And the solenoid core is simply held in by a screw, here is the final design in place, notice the screw at the bottom:
The electronics for controling the solenoid is simple. I used the example at Arduino's own site (LINK). But before I would hook the targets up to the Arduino I wrote a test program to turn each of 5 outputs on and off. I tested this with an LED hookup:
Once that was tested I could replace the LED's with the transistor circuit as described in the Link above, and hook up the outputs to the targets. (Which were now held together on a strip of 1x2 wood.
Next I would have to add something to detect when the target was down.