I think Rodney won't claim to be an expert in anything until he's very very sure he is.

He disparages his own skills at flying in Epiphany, though its been a year and he could easily have learned a lot since then.

I think guns and flying were just not skill sets he ever imagined learning, nor were they things that simply dropped at his feet as EASY and that is very new to him. Having to try and learn for these things in completely different ways than he had to for the science and the math.