I am going to take a stab (pardon the pun) at expressing my mindset on this. I am not an expert, and I won't claim to be. This is just an attempt to express my mindset.
Somewhere along my travels, I came across the concept of "threat zones." My thoughts employ this concept.
Zone A: Torso to torso contact. This is the groundfighting / grappling zone (though not necessarily on the ground). Short, fast strikes, like throat grabs and eye gouges work best here, as do blood and air chokes. Edged weapons are especially dangerous here as you are "too close" to see them until it is too late.
Zone B: Well within grasp. You can reach them with an elbow stike. Elbow, knee, and palm strikes work best here. You may or may not be "clenched" so grappling still works here, but this is primarily a "striking" zone. Edged weapons rule the roost. Pistols work, as can close combat with rifle or shotgun.
Zone C: Arm/ leg length away. This is the long punch and kick zone. Firearms may also work here as do edged weapons. This is the a transitional zone, where you are either moving into their B zone or trying to push them into your D zone.
Zone D: The moving in/ moving out zone. You are not in contact and you have to move in to make contact. This is the primary handgun zone. Some might call it 'spitting distance.' Again this is a transition zone, hence the "moving in/ moving out" moniker. Bayonets shine here.
Zone E: Target ID is easy (close visual range) but actual physical contact is not emminent. This is the convention "handgun zone" in most people's mindset. Shotguns and rifles start to become better options. This is the 7 to 50 yard zone.
Zone F: Target ID is possible, but more difficult. Shotguns without chokes are not as effective. Handguns are largely useless (to the average shooter/ BG) except as a deterrent. Slug equipped shotguns and rifles rule the roost. Pistols are a "trained operator" threat only, besides a "lucky shot." 50-200 yards.
Zone G: Target ID is very hard without optics. Rifles are the only viable option. Optics make target ID much easier. Depending on shooter and rifle this can extend to a half mile and beyond.
Zone H: Out of range. Threats are not within a range that they can be positively IDed as such. Alert, but not engaged... unless you are a mortarman or artilleryman.
This is just MY framework. It acts as a 'training guide' for me as I ponder and practice transitions to and from these "zones." It highlights why I believe no weapon is the "end all, be all," and illistrates why I believe a well-rounded approach is best. As I said at the start... I am no expert... this is just MY thoughts, born out of MY experience.
P.S. If you find fault in my reasoning, please let me know. I am always ready and willing to learn better ways to defend myself and my loved ones.