Best gun to use
I found that while playing Pterattack2, the best gun to continually upgrade is the Fire ball (the red one).
They are a lot less common than the other guns, so I suggest that you either upgrade the basic blue gun once, to wipe out Pteradons that are in your path much more efficiently to therefore increasing your chances to acquire the Fireball as you will stay in the game longer.
Another upgrade that will allow you to wipe out your opponents quickly enough to receive upgrades to the Fireball is the Pterboom: It’s a purple spinning powerup. Like the Tadow in the old game. It’s a boomerang that, rather than coming back, weaves across the screen so you won’t need to be directly underneath the Pteradons (of any level) to shoot them.