If the situation is so bad that you will need a rifle, there is a good chance that the other guy started things off by shooting at you. When you are ‘trading shots’, it pays to maximize them as a target and minimize yourself as a target.
Suggestions for the caliber for the ‘only gun’ … IE, the only gun that you own or practice with regularly include 223 Remington and 308 Winchester. People talk long and hard about the differences between the 223 Remington and the military version called 5.56x45mm NATO just about as much as they talk about the difference between 308 Winchester and 7.62x51mm NATO. I am in the business of testing ammunition and I have shot 223 Remington in 5.56mm chambers and vice versa many times. Same with the 308 Winchester. Nothing negative has happened yet. The benefit of using military cartridges is that you can shoot military projectiles. If the current direction of integrating robotics into military combat roles continues, having the ability to pick up and shoot armor piercing ammunition may prove a godsend in the future.
One of these days, if I get lucky and get to move to a place with wide-open terrain, and dangerous animals (four-legged) living all around me, I will quit shooting my AR-15 and buy a 308 Winchester semi-automatic rifle. While there are many good rifle calibers out there that will fill the same needs, I like the 308 Winchester for the relative ease of finding ammunition, the intermediate ‘power’ of the cartridge and the ability to shoot out to 600 meters (with enough practice.) If the target shows up close, you can meet it with twice the kinetic energy of the 223 Remington cartridge.
Rifles give you the ability to cleanly take down game animals for food (given proper shot placement and ammunition type), defend yourself and just practice the fine art of rifle marksmanship. Contrary to popular belief, rifles can be fired indoors but I assure that the effects must be experienced first-hand to be properly appreciated. A 223 Remington is about all of the blast that I can stand—larger rifles can be used for home defense but the muzzle blast effects are something that you should experience before relying on a rifle for home defense. Rifles of 223 Remington and larger will pierce all soft body armor vests with any type of standard FMJ ammunition. That’s important to know before using surplus vests as a backstop … I’m sure!
There is much said about scopes for rifles. I suggest that if you want to use a scope, that you buy the best scope that you can afford. Money equals a quality image to a large extent when discussing optical gear. Avoid high magnification scopes unless you actually are a long-range shooting practitioner. To help myself when selecting a scope magnification and objective lens diameter, I first ask myself “am I as good a shot as a US Army sniper in Vietnam?” The answer always comes back a resounding “no.” I jest, to make a point. The Unertl MST-100 scope is a popular sniper scope with the US military. It is a fixed 10-power scope, with a 40mm objective diameter which gives you a constant exit pupil diameter of 4mm. The average adult eye will dilate to 4mm diameter when fully adapted to dark conditions. During the day, the light gathering ability of a scope doesn’t matter nearly as much as it does if you find yourself shooting at night. Picture the situation as similar to burning paper with a magnifying glass—as you move the glass closer or further away from the paper, you increase or decrease the diameter of the circle of light that the lens is projecting onto the paper.
For me, I don’t use a scope at all on rifles. Scopes usually are fragile, expensive, subject to fogging, have electronics in them that are vulnerable to solar flares or require batteries. There are some notable exceptions to this among the military-grade red dot sights but it pays to shop around and be absolutely sure that you can manipulate the scopes operating surfaces under stress. If I had to use a scope, I would use a military-issue red dot scope. Scopes that work on other spectra such as night vision and infrared scopes—are worth their weight in gold and I highly recommend you consider obtaining one or more of these such devices.