You can’t actually load a semi-auto pistol with a magazine in place, or, you can’t without removing the slide and barrel. Most pistol magazines are double stack, single feed, so the bullet needs to start in front of the magazine. (I think that Steyr makes a double stack/double feed pistol magazine?) But with a magazine stuck in place, the barrel is going to be in front of the bullet, leaving you no room to load the magazine.