When you drop a single 500 pound'er, you need to be nearly dead-on, on heavy tanks. Two 500s or a 1000 pound'er will give you more room for error. The point is, if you stick with a single 500 pound'er, you need to practice quite a bit. I know a couple of players that must be near 100% with a single 500 pounder with the tank moving or not.