They should have set that up earlier, like show him sparring with another trooper or at least have him armed with one of those stun blade things so we know he can handle himself.
I don't feel it really matters. We see early on that Rey is great with her staff, but later when Han Solo gives her a hand blaster, she figures it out and has pretty good accuracy too. In the same way, Finn is more comfortable with a blaster because that's the weapon he's mostly trained in, but he can handle a melee weapon if needed. The point is that these are young people but they're ones who have experienced a harsher upbringing and know how basic weapons work.