It is the boom extend that is dropping, looks to me like it collapses all the way, which is why the oil stops spraying out. The hydraulic cylinders that I have taken apart don't seem that thick, and I believe a lot of bullets would be able to put a hole into them. I have seen where some one has shot at a 4 inch corner post that is 3/8" thick steel and there is a bullet hole in it.  Most hydraulic cylinders are not that thick.