[555 Based Flyback/Ignition Coil Driver]

audioguru is right.  The biggest problem working with high voltage and/or high current is zapping you transistor.  It's not hard to build up spikes that will kill your transistor.  A diode in reverse polarity across the coil is a good place to start.  If your transistor gets zapped, a snubber circuit may be needed.  Working with high voltage/high current can be a very hit and miss project.  Have fun.

[555 Based Flyback/Ignition Coil Driver]

What everyone is saying is that a 555 does not put out enough current to drive the power transistor into saturation (all the way on).