What kind of rod are you using for the firing pin? Black or clear rods are less flexible so they don't bend nearly as much as a normal grey rod. That's my first recommendation. Secondly, check the placement of your bands. It's best to make attachment points on the exact same level as the barrel. If you can't do this, try to make attachment points on the top and the bottom of the barrel the same length away and hopefully they'll balance out. Thirdly, if it's your own gun you're building, I would recommend adding a pin guide. I don't really have an example on hand.