I think 300 dollars is probably all its worth now...
There is no way that the rust will come off w/o altering the finish in some manner. After removing the rust the whole gun can be cleaned with the SemiChrome. The amount of shine/polishing achieved is dependent on how much effort you put into it. It's not going to shine with one application of SemiChrome. If you want a more satin finish you can use polishing pads of various coarseness' until you get the desired effect.