Something has gone terribly wrong and you become aware that a stranger is in your home. What to do?
The most important thing is the safety of you and your family. The most likely way for someone to get hurt is to have a face-to-face conflict. Convincing an intruder to leave without coming into face-to-face contact is always best.
You should have someone dial 911 as soon as it is safe to do so. Even if you don't have the opportunity to speak, leave the line open. The operator will listen to what is going on while dispatching an officer to investigate. This puts a time limit on the situation; if the intruder does not leave shortly, law enforcement will be there to assist. If you are armed, or you believe the intruder is armed, you should inform law enforcement. This speeds up response times and helps law enforcement to know what to expect and avoid accidental tragedies.
Under Indiana law, you do not have a duty to retreat before using force to defend yourself in your home. Additionally, you do not have to determine whether the person intents to harm you before using force. HOWEVER, this is not the Wild West. It is far better to convince the intruder to leave than to have an armed conflict. If an intruder is shot in the back, it is entirely possible that the law will believe the intruder was retreating, thus no longer posing a threat, and you would potentially be subject to criminal and civil sanctions.
The decision whether or not to own a firearm is a very personal one. Among the things to consider should be whether or not you believe you can shoot someone. The taboo against killing another human being is so strong, historically, 25 percent of solders never fire their weapon at the enemy and another 25 percent either chose to not aim or intentionally fire above the enemy's head. Displaying a weapon you cannot or will not use may result in the intruder taking your weapon or using his own against you. If you chose to own a firearm, it should be something that you are comfortable using and you should practice at least until you are proficient with it.
The number two rule with firearms is to be aware of what is behind your target. (Number one is to always treat a firearm as though it is loaded.) The higher power the weapon, the more likely the projectile will travel past the target striking whatever is behind. Walls may look solid, but the vast majority of them have large empty spaces within and are relatively easy to penetrate. How tragic would it be to fire a weapon at an intruder in the hallway and kill your child sleeping in her bed or the neighbors' child in her own home?
This is not a time for trick shots. You should not try to shoot at the head or feet. In a stress situation, the changes of missing your target are tremendous. You should always aim for the center of body mass. In real life, the purpose for using a weapon is to make an opponent disengage from conflict. A hit in the center of mass is most likely to do that. If you miss the bull's-eye, you retain increased odds of striking the intruder in a different location.
If something can go wrong, it certainly will. This is especially true during times of high stress. That means, "More simple is more better." The two best choices of firearm for home protection are thus the most simple and reliable to operate: The pump action shotgun and the revolver.
There is no other sound in the world like someone racking a shotgun slide loading a shell into the chamber. This sound should give an intruder a sudden urge to leave your home and change his shorts. If the weapon must be fired, a shotgun makes a boom louder than most other weapons, which both intimidates adversaries and alerts neighbors.
Whether 12 gage, 20 gage, or 410, the weapon should be loaded with skeet or light bird loads. Slugs, buckshot, and other heavy loads are likely to go through several walls striking who or what ever is on the other side. Additionally, use of a shotgun does not relieve you from the obligation to aim the weapon. The pellets spread much less than you think; especially at short ranges (like within your home).
If you rely on a handgun, it is my opinion that it should be revolver, rather than an automatic. An automatic needs to have the slide operated to put a round into battery. Under stress, this can be forgotten or difficult to do. Additionally, no matter what any manufacturer or your buddy says, all automatics are susceptible to jamming. Finally, the vast majority of loads for automatics are high velocity giving rise to the issue of over penetration and hitting beyond your target.
My personal bias is toward "heavy and slow." Heavy slow bullets are less likely to over penetrate. This also means that they are more likely to transfer all of their energy into the target they strike. My personal recommendation would be to use either a .44 special or a .38 special, preferably with light charges. Additionally, using "hollow nose" expanding bullets also increases down range safety. As the bullet expands, it sheds energy into the target at an accelerated rate and is less likely to exit a target and continue down range.
Hopefully, this is information you will never need to know. However, as the St. Louis store clerk said to people starting down the Oregon Trail, better to have it and not need it than need it and not have it.