A dolly is like a auto body mans tool that acts as a backup on the other side of a piece of sheet metal to help you form it. If your magazine is crushed you will need to reform it back to it's original shape. Be careful on how you hit the magazine, it doesn't take much to distort and stretch the metal.

If you look at the cartridge feed lips on a Mk 3 they are retained by a hook at the bottom of the mag and for lack of a better word a dove tail towards the receiver. Remove the trigger guard by removing the bolt from the rifle to access the rear trigger guard screw and unscrew that along with the front trigger guard screw in front of the magazine. The trigger guard along with the magazine should now come free.

If the mag is not too distorted you should be able to remove it from the trigger guard. Sometimes it can be a b!tch. Once you get it out, carefully pry up the hooks at the bottom of the feed lips and push them up towards the top of the mag until they release from the dove tails and remove them.

They are made from spring steel so they should not deform. Using a straight edge check your magazine, it should be flat front to back and top to bottom.