Tank bouncing can be tricky... there are a coupla things to know about it
1) it only works with the green type of tank, because the green one is the only one with the proper bounce areas
2) you have to hit on the slanted parts of the tank on teh back side. If you look at a tank, the back has two areas that are slanted. You must hit one of those areas
Sadly i think that very 1st window will be patched because it makes for a big glitch just off that one building where you can go from building to building. treyarch will patch that i bet.