The first problem is having a gun that you have never used. Without preparation, you have already stacked the cards against yourself. If the intruder is armed, you are basically in a drawing duel and if you don't know how to operate your gun, like turning off the safety or racking the slide to load the chamber if it is an auto pistol, your probability of getting shot first is higher. Even if he isn't armed, there's the possibility of your gun not going off when you pull the trigger. Your intruder can run away or can attack you during the moment of "Uh oh". Practice is important. Your intruder may be farther than point blank range, e.g. maybe 15 feet, and you can miss a target at 15 feet.
Second problem, you don't start dumping rounds. The person who came in your home might not be an intruder but a roommate, friend or family member or someone who you know with access to your home.
Third, and it goes with the first problem. You have to practice with your firearm and you have to have a plan in case an intruder comes in. You need to be tactically smart because you might have professional burglars. They may be armed and are prepared for armed homeowners.
They may practice often with their firearms so that you, standing in the middle of the room are an easy target.Also, you don't want to mag dump all your rounds into one guy. There might be more guys in your house. It is not unusual for burglars to work in pairs/groups.
Fourth, know the laws in your state. What you can and cannot do, more specifically, when can you shoot and when can you not shoot. Because what is going to happen after it's all over is the cops are going to come over and take all your statements. Forensics are going to come in take photos and stuff. If your story doesn't match up or your story puts you in a position to be arrested, then you will go to court as the defendant, even if you were the homeowner and the intruder did break into your home. Most likely, this would involve intruders running away, and you shot them while they were running away. Without going into details, if they split and you chase and they are running away from your home without weapons in their hands and you shoot, then that makes you look bad. There's gray area though in different circumstances. For example, as you started shooting, the intruder turned around quickly and was hit in the back.
You don't want to freeze up. Just like in any other dire situation, that can screw you over. You can get shot if the intruder is armed if you have a "what am I going to do?" moment when he pulls his gun out because you didn't think he would have a gun.
Warning shots? No. You could damage your home, but more importantly, you could injure someone downrange of your warning shot, e.g. other occupants of the home or apartment.
Shooting at the legs? Stupid. You could miss and if your intruder is aiming at your body, guess who is going to win that shootout?
