What voltage/current does the detonator require?The way to set it up is to have a resistor on the arduino pin going to the base of the transistor. Hve the emitter go to ground with the detonator in series with the collector(ground wire of the detonator to the collector pin).
Any TO-220 type - MOSFET or NPN use 4 A or more.
A SCR rated of more than 4 A, is better for your application. I agree with BulletMagnet83 on this one.The TIP122 is OK too.
Post pics of yours when it's done Would be interesting to compare notes. I'm in the process of miniaturising mine now, so it can be used in a hand-held "stick it to the wall, set it and run" sort of fashion. Wondering if I should use the external interrupt features to devise some sort of anti-handling feature.... mmm naughty. ]Just checked my datasheets and the TIC106 I mentioned can handle 30A surge, 5A continuous. Might be worth considering for future projects as you'll find it pretty difficult to kill one in your chosen application!