Picking the right ammo to carry for defensive purposes can be a hassle. There are many publications like Ammo & Ballistics, along with other defensive pistol magazines that go into extreme detail on this subject. My short answer is this:
Pick the largest round you can shoot accurately.

Shot placement is key regardless of the size of the round. Now for defensive purposes you should already consider the JHP or jacketed hollow points. This is a given due to their ability to expand and have a maximum affect on the target. Glaser Slugs are a special consideration for those who have their firearms for house protection only. Those that carry should focus on the JHP.

Now for the infamous question, what size round should you use? Anything above a .38 special. Yes, the government and many others have tested many different calibers and you will hear a lot on that data, but as someone with military service, I can tell you they just used full metal jackets. Why? Because that is the only type of ammunition we can use in the military.

The terminal ballistic performance of calibers .38 special to .45 ACP  in JHP are virtually the same in one aspect, the secondary wound channel. Yes, they all have different speeds and energy levels, but we are not talking about the Knock Out Factor or anything crazy here. Just the affect the bullet has in the human body as related to what we military people call hydrostatic shock.  These hollow points expand under perfect conditions and will have the desired affect on the target. Trust me, the target will not care if it is a 9mm or .40 S&W.

As a person who carries their firearms for defensive purposes you should know that you are fully liable for that round you let loose. So practice will always be important. You accuracy is number one on the priority scale. Second to that would be for you to test your chosen JHP in your firearm. Of course I can perform a reliability package on your firearms and you will not have to worry about feeding problems, but a lot of factory pistols will be picky on what JHPs they feed reliably. Test at least a hundred rounds of your defensive ammunition.

For those who carry a .380 ACP, the round will still do the job. However, it is designed for a back up or light concealed pistol. The .380 ACP just does not have the power to expand and cause a drastic secondary wound channel. All rounds will kill a target, but for a defensive pistol we want that target down immediately. Always prepare for the worst and expect the best. So stick with a .38 special and above.

Safe shooting

These rules should be fully memorized in both your mind and body. No matter if you are a novice or an expert, all should practice these rules all the time to prevent accidents.

1. Treat every weapon as if it were loaded. Even after they have been ensured that they are unloaded.

2. Never point your weapon at anything you are not willing to destroy.

3. Keep your finger straight and off the trigger guard until you are up on target with the intention to shoot.

4. Keep your weapon on safe until you intend to fire and be sure of your target, its background and foreground.

Safe Shooting.

A good way to prevent rust from accumulating on your firearms in your safe would be to use Silica Gel packets.

These packets are found in a brand new pair of shoes, clothing, and almost anything shipped or bought brand new. On the packet it says "Throw Away Do Not Eat." Well I agree that you should not eat it, but do not throw them away. The Silica Gel is Silicon Dioxide or (SiO2). These little balls of gel are very porous and can absorb as much as 40 percent of its weight in moisture. Or in an enclosed container, it can bring down the humidity to a about 40 percent. You can reuse them when they get saturated  by simply re-heating them to about 250-300 Degrees Fahrenheit for about 30 minutes or longer depending on the size of the gel ball. If you want to see how well they work, put a couple packets on your dashboard of your vehicle at night and notice that the fog on the windshield that is usually there in the morning will be gone.

Of course a good coat of oil like Break Free CLP will help, but always keep in mind that moisture will cause rust on metal. There is always moisture in the air as well as in the gun oil itself. Using the Silica Gel packets is an easy, cheap, and effective way at preventing rust in your gun safe.

Any firearm, no matter if is shot frequently or rarely, should have an annual cleaning and inspection on all parts. This ensures that the firearm will be reliable when it is needed.

When a firearms sits, the oils and residue collect dust and other microfibers. In addition to that oxygen has the habit of creeping in small places and rust occurs. Having an annual cleaning and inspection includes a full disassembly, thorough cleaning, close up inspection, oiling, and reassembly.

Keep your firearms in full operational order by having an annual cleaning and inspection done.

Duty firearms or firearms that are shot often should have this procedure done semi-annually.