Since the rise of Concealed Carry laws in the 1990's, .380 acp has been enjoying an resurgence in popularity. Ruger just produced their Kel-tec look-a-like, the LCP this past year. Walther has redesigned their popular P22, in a larger size to accommodate this century old caliber dubbing the new polymer framed pistol the PK380. Even Sig has gotten into the swing of things. Pictures of new Sig p238, a virtual clone of the Colt Mustang, have been plastered all over forums and blogs since the last ShotShow.
The .380 acp or 9x17 Kurz(short) (as it is often known in Europe), was found in the magazines of many compact pistols favored by police and military in the early 1900's. It was to Europeans what the .38 Special was to the United States. It is not a powerful cartridge. It was designed to be used in blowback semiautomatic handguns. The blowback system uses a frame mounted barrel, and only uses springs to retard the effects of recoil on the slide and return the slide to the ready position. This system is very simple and cheap, but limits the power of cartridges that can be used. At its best is only roughly on the same power level as standard pressure .38 Special. The .380 is considered by most to be the dead minimum cartridge for serious defensive work. Despite this, a plethora of handguns were chambered in it. Its small size, low recoil, and ability to be chambered in the early 1900's smallest fighting handguns made for great success. Its position as a dominant military and police caliber was gradually encroached and supplanted by the more powerful 9x19 parabelum cartridge.
The .380 acp has enjoyed a lot of resurgence after the CCW laws started passing in the 90's. Both new and classic designs have done very well. New technology has worked to make .380s smaller and lighter to satiate the massive public demand for lightweight, easily concealable handguns. However, many of the same technological inroads have started to apply to more powerful calibers. 9x19mm started to gain serious ground with subcompact and compact designs like the Glock 36 and 19, Kahr's pm and pk lines, and even tiny allow 1911s from various companies. However, despite the power and cost advantage 9x19mm holds, the .380 isn't slowing down. Why choose a less powerful .380 when there are 9mm's in the same size factor? Cost, weight, and ergonomics.
Cost: .380 is definitely less powerful, even in +P versions. However, that lack of power is not a weakness in all areas. That lower pressure cartridge does not need the same level of structural strength in a pistol that a 9x19mm will. Aluminum alloy and plastic can be used more liberally, and are easy materials work with in comparison to steel. Also, the recoil operation need not be as complex. A 9mm in the same size may need much more complex operation.
Weight: Once again, less and lighter weaker materials can be used in your average .380. With the lower recoil, it can still be effective as a weapon at this lower weight.
Ergonomics: I can hold and use a full size pistol like a sig 226 without troubles, but my wife can barely use it in single action. Many CCW holders are women. Not all of the male holders have big man-hands. Double stack compacts and subcompacts are fine, if you can comfortably use it and conceal it. I can think of a few single stack compact and subcompact handguns of decent quality and modern manufacture in 9mm para off the top of my head. The Kahr 9 series(TP9. T9, CW9, K9,P9,MK9,PM9.), the Sig-Sauer p239, taurus PT709, Sky CXP-1, Kel-tec pPF-9. The Taurus hasn't even hit the shelves yet. I can name off only a few of double stack .380s. The Beretta Cheetah 84, cz-83, Taurus 138 millium, Taurus 58, Taurus 93, Bersa thunder 13rd, and the Glock 25 & 28(neither of which are sold to the American public). I can name a lot of single stack .380s. Their numbers seem to be growing. Also single stack designs tend to be flatter, and conceal a bit better. Especially in the butt. Heh.
The .380 is going to keep going strong until the big names in the 9mm business decide to make some handguns in similar sizes that work well. The .380 killer needs to be 20oz or less, single-stack, and at a $400 or less price point.