It seems logical that anyone who keeps a firearm handy for defensive use would practice with that weapon to ensure that if they have to use it there are no surprises. This means practicing at close range (3 yds or less)- not 7 yds, not 25 yds and certainly not 50 yds. If you have an AR platform rifle or pistol as your primary home defense weapon or even as a potential option for home defense you owe it to your loved ones and yourself to practice close quarters handling and firing.
Bullet trajectory is an interesting phenomenon. Some good background on trajectory is here. If you want to be good at using your rifle in places other than a square range with targets at fixed distances take a few minutes to learn about what happens when you press the trigger.
The photo above illustrates a traditional sight picture (what is used for a typical range shot at a target 25 or more yds downrange). The chart in the trajectory article referenced above illustrates where a close quarters shot will go with pretty much any zero – you’re going to hit high low (thanks to D. Russell for catching and pointing this out) if you aim the way the photo above depicts. In fact you may miss the target entriely or fail to neutralize the threat- and neither result is acceptable when the safety of you and your loved ones is at risk.
This problem has a pretty easy solution- one that is reliable and repeatable enough for a wide range of people that the US Army teaches it. The proper method of aiming close quarters shots is to maintain the typical rifle shooting position (cheek weld, nose touching the charging handle, etc.) but to look over the rear sight and use the front sight the same way you always do.
The photo above illustrates this close quarters sight picture. Basically you want to position the base of the front sight on top of the ghost ring. The technique is reliable up to about 13 yards (12 meters) and allows you to engage threats quickly with the sort of accuracy required for self defense. In other words this should cover most home defense situations.
Be sure to get out to a place you can practice this- that’s one key to deliberate action in a life threatening situation. Also be certain that you are using ammo that is safe for home defense- Range ammo (FMJs), Most frangible ammo and even Defensive ammo like Hornaday TAP can easily exit the walls of your home and keep traveling. Be responsible and stay safe out there.