Get a small plastic mallet from Canadian Tire. The main spring will eventually loosen up but the mallet comes in handy when loosening the barrel and a bunch of other stuff you never thought you needed one for. Also depending on which manual you have there may be an error in it. To reinsert the mainspring housing the mag must be inserted first, otherwise it gets tough and many never clean their MK III again because of that.
By the way watch you don't hit the sight when bashing the mainspring, easy to break.