This seems to have become a game of chance rather than skill. I've tried aiming, I've tried shooting at random, and sadly, I don't see a difference in the scores I receive. And on some shots that do hit the bullseye, it tells me I only hit for +5 even though the hole left behind is clearly in the center. Then again, it occasionally gives me a bullseye for hitting the +5 ring.

EDIT: Well, I just went back and played again, and discovered that a new balloon was added as well (unless I have just never seen it before):

It gives 2x the bonus of the normal balloons. Another discovery is that if you save all your powerups until your last shot, it all gets wiped out if you are unfortunate to get Punchbag Bob as a target. I had three powerups selected for the last shot, Bob was put in and *poof*! all my powerups were GONE.