Forget the voltage, it's the current you should be worrying about. And the Arduino isn't going to give you nearly enough current for ignition. Here's a really good article that actually measured these ignitor currents. A "standard" Estes ignitor (Figure 11 in the document) needs a burst of current well over 10A to do its job, and that's just not coming from an Arduino.http://www.gwiz-partners.com/igniters.pdf
Now, you can use the Arduino to switch a relay or MOSFET that in turn conducts current from a battery pack that can source that instantaneous current. Then you can easily have your 6V too.
The Gadget Shield
: accelerometer, RGB LED, IR transmit/receive, speaker, microphone, light sensor, potentiometer, pushbuttons