It should be well-known that when looking for ammunition to be used for personal defense in a handgun, you should use some sort of expanding bullet. The most common type of these are jacketed hollow points (JHPs), and they are preferred to full metal jacket bullets (FMJs) because as a mechanism of their expansion they transfer more energy to the target. Also, because they expand, there is much less chance of them over-penetrating.
Just about every ammunition manufacturer offers some sort of JHP round for every caliber they make. That said, this is America, so many ammunition makers are not satisfied with just making "hollow points"...so we have Hydra-Shoks, XTPs, Gold Dots, and the famous, eeeeeevil Black Talons.
Hornady is currently offering two lines of hollowpoints with similar names- Critical Defense and Critical Duty. There's a difference, but I don't know if Hornady is doing that good of a job explaining exactly what the difference is between their two ammunition lines. Let me try, and I'll start with the newest first.
Hornady's Critical Duty ammunition is high-performance full power hollowpoint ammunition not just designed for full-size duty weapons but designed to pass the famous and stringent FBI ballistic protocols. What are those protocols?
The FBI protocols are a specific set of tests for handgun ammunition where they are fired through a number of intermediate barriers into ballistic gelatin. Penetration is the only fixed requirement- if the bullet doesn't penetrate a minimum 12" of ballistic gel after passing through the barrier, it doesn't pass. Expansion isn't required, but the more expansion a bullet shows (provided it penetrates the required distance) the better.
There are eight test events:
1. Bare gelatin.
2. Through heavy clothing into gelatin
3. Through 2 pieces of 20 gauge steel (sheet metal) into gelatin
4. Through wallboard into gelatin
5. Through ¾" plywood into gelatin
6. Through angled automobile glass into gelatin.
All of the above tests are done at ten feet, and the heavy clothing and auto glass tests are done again at a distance of 20 yards for the final two tests.
The FBI protocols are used by just about every police department in the country when it comes to picking out duty ammunition- if your company's ammo doesn't pass, they probably won't buy it. That said, any ammunition which can pass these tests tends to have stout recoil, and is best suited for full-size guns. It also tends to be loaded with very tough bullets which will definitely penetrate, and probably expand.
With their Critical Defense line of ammunition, the engineers at Hornady realized that most citizens looking for ammunition for their concealed handguns aren't likely to be shooting through sheet metal or auto glass, and will most likely be carrying smaller, lighter guns. Making ammunition for this target market didn't require slavish devotion to the FBI protocols, because the needs of the private citizen were different. Also, most carry guns have shorter barrels, and so this ammunition is optimized for use in short-barreled pistols.
Small, light guns, especially those small enough to fit in pockets, are very convenient to carry, but loading them with hotloaded +P ammo originally designed for big heavy guns can make them tough to shoot. Considering the circumstances in which most concealed carry guns will be used (conversational distances, with little or no intervening barriers), sometimes less is more.
Both Critical Duty and Critical Defense rounds are tipped with what appears to be Hornady's FTX bullet, but that's not the case. The Critical Defense rounds are loaded with the FTX bullet, which has a flexible polymer insert in the "Flex Tip" hollowpoint cavity which resists clogging when flying through thick clothing and/or drywall, and yet initiates expansion when it actually hits someone. The Critical Duty line is loaded with the "FlexLock" bullet. The FlexLock has the Flex Tip point, but it is paired with their InterLock band which locks the jacket and core together. This bonded core stays together better when going through intermediate barriers (i.e. the FBI protocols).
The Critical Duty line of ammunition is still new, but even so, it is aimed more at the law enforcement market than Hornady's other lines of hollowpoints. I'm guessing it will never be offered in that many calibers. Currently Critical Duty ammunition is only available in 9mm, 9mm+P, and .40 S&W. The Critical Defense line is offered in 12 different calibers, starting at .22 WMR- any caliber you're likely to use for self defense.