Can you shoot 380 ammunition in a gun chambered for 9mm Luger?
The short answer is yes.
The long answer is yes, but...
[BRIEF HISTORY OF 9mm LUGER]
In 1902 George Luger designed the 9x19mm Luger cartridge, which now bears his name. German weapons manufacturers chambered the now famous "Luger" pistol around the cartridge. It is a "rimless" cartridge, meaning that it lacks a "rim" at its base which exceeds the outer diameter of the case above it. It is now the most popular pistol cartridge in the world. Variants of the cartridge are in use by the militaries every major nation in the world and NATO has standardized on its own variation, differing only in the internal combustion pressure and bullet weight from George Luger's original design. Because of its popularity and great international usage, there are a vast number of firearms available to the civilian market chambered in 9mm Luger. For the same reason, 9mm Luger ammunition is also among the most commercially available and least expensive. Once considered a mediocre, or even poor, defensive round, advances in bullet technology, driven by the popularity of the round, have improved the performance of the cartridge to a level where most firearms self defense experts endorse the cartridge, "with the proper ammunition choice," of course. Common designations, slang, and variations in the U.S. include 9mm, 9mm Luger, 9mm Para (9mm Parabellum), and 9mm NATO.
[BRIEF HISTORY OFF .380ACP]
The .380ACP (.380 Automatic Colt Pistol) was designed by legend John Mosses Browning in 1908. It is a "rimless" cartridge. Its design intention, and first offering, was for the Colt Model 1908 Pocket Hammerless, a popular "vest pocket" pistol in its day and still widely regarded as an ground-breaking design with excellent ergonomics and a gun particularly well suited to "pocket carry." The .380ACP has always been a popular cartridge in Europe but experienced decreasing popularity in the U.S. following WWI and WWII, as the shooting population of the nation gravitated to larger, heavier, or more energetic cartridges, typified by the .45ACP of 1911 (and also John Mosses Browning) fame. While the .380ACP still maintained a small following in the U.S., particularly in sub-compact pistol sizes as was exemplified by the Walther PPK, the popularity of the cartridge did not really rebound until Kel Tec launched the "pocket 380" craze with the introduction of the Kel Tec P3AT. Now nearly every major, and most minor, handgun manufacturer offers a .380ACP chambered gun with dimensions similar to that of the P3AT "pocket rocket." This resurgence has similarly spurred dramatic improvements in expanding point .380ACP ammunition, far surpassing performance characteristics of the old .380ACP FMJ (Full Metal Jacket) "ball" ammunition of yesteryear. In Europe, the cartridge is variously referred to as the 9x17, 9mm Short, 9mm Kurtz, 9mm Corto, 9mm Browning, or some variant of those.
Case type :- Rimless, tapered
Bullet diameter :- .355 in (9.01 mm)
Neck diameter :- .380 in (9.65 mm)
Base diameter :- .391 in (9.93 mm)
Rim diameter :- .392 in (9.96 mm)
Rim thickness :- .050 in (1.27 mm)
Case length :- .754 in (19.15 mm)
Overall length :- 1.169 in (29.69 mm)
Maximum pressure (C.I.P.) 235.00 MPa (34,084 psi)
Maximum pressure (SAAMI) 241.32 MPa (35,001 psi)
Bullet weight :- 90 gr. - 147 gr. (originally 115 gr.)
Case type :- Rimless, straight
Bullet diameter :- .355 in (9.01 mm)
Neck diameter :- .373 in (9.5 mm)
Base diameter :- .374 in (9.5 mm)
Rim diameter :- .374 in (9.5 mm)
Rim thickness :- .045 in (1.1 mm)
Case length :- .680 in (17.3 mm)]
Overall length :- .984 in (25.0 mm)
Maximum pressure :- 21,500 psi (148 MPa)
Bullet weight :- 85 gr. - 100 gr.
[9x19mm Luger on left, .380ACP on right]
Many of these dimensions are comparable, some are different, and a few very important ones are identical. The most important of these is bullet diameter. Both cartridges use a bullet of exactly the same diameter. This is important because if a bullet with too great a diameter is forced down a barrel with too small a diameter it causes a dangerous pressure spike which could cause the firearm to fail in a dramatic and potentially injurious way. If the bullet is too big the gun could go "kaboom." However, the .380ACP and the 9x19 Luger use bullets with the same diameter. The .380ACP bullet weight falls within the range in use by the 9x19 Luger and the maximum pressure generated by a standard specification .380ACP falls under the maximum allowed pressure for a gun chambered in 9x19 Luger. Further, the case diameter and case/overall length of the .380ACP allow it to "fit" in a 9x19 Luger chamber, even if it is 0.007 inches (0.05mm) "loose" in diameter and about 0.1 inches shorter. Sounds "safe" enough.
However, there is another very important measurement: the case length. The 9x19 Luger is roughly 2mm (~0.1 inch) longer. This is vitally important because of something called "headspace."
[HOW 9MM AND 380 "HEADSPACE"]
What is headspace? According to the Sporting Arms and Ammunition Manufacturers' Institute (SAAMI), headspace is the distance from the face of the closed breech of a firearm to the surface in the chamber on which the cartridge case seats. (emphasis added) Take note of the emphasized part: "on which the cartridge case seats." With many straight-walled, rimless, cartridges, such as the .45ACP, the cartridge seats on the "shoulder" by stopping up against it with the leading edge of the case. The shoulder is sort of like a shelf or step dividing the chamber from the barrel. Both the .380ACP and the 9x19 Luger were designed to headspace at the shoulder. In other words, when the cartridge is fit into the chamber of the gun, the top of the case stops up against the shoulder of the chamber. This prevents the bullet itself from intruding too far into the barrel, which might dangerously engage the rifling, and uses the strength of a machined and heat treated solid steel part, encasing the total circumference of the cartridge, to maintain proper alignment and control of the mini-grenade we call an ammunition cartridge.
[Chamber and Shoulder]
[BUT IT WORKS, RIGHT?]
Yes, you can often load a .380ACP cartridge into a firearm chambered for 9x19 Luger and fire it without the gun exploding. But there are some pretty important caveats and drawbacks to consider.
First, because the .380ACP cartridge is ~2mm shorter, and is not being seated properly ("headspaced") on the chamber shoulder, when fired, the bullet must "jump" those 2mm to the barrel. This "jump" can contribute to chamber wear and gas cutting, particularly at the shoulder itself. As that shelf gets worn down, more rounded, it is less and less able to perform its task of "seating" a cartridge when a proper length 9x19mm Luger cartridge is inserted. Over time, this could mean that the bullet of a 9mm cartridge intrudes too far into the barrel, engaging the rifling too early, causing dangerous pressure spikes. Additionally, as we discussed above, the case diameter of the .380ACP is slightly smaller than the 9x19 Luger. This extra room can contribute to gas cutting, additional wear, and shortened life of the chamber.
Second, if the .380ACP when loaded into a 9x19 Luger chamber isn't headspacing on the shoulder, what exactly is it headspacing on? The answer is "nothing." The extractor claw is holding on to the case at the extractor groove. At best, this tilts the more narrow case diameter cartridge in the chamber and misaligns the bullet to the barrel, causing more chamber and shoulder wear. Additionally, because the cartridge isn't headspaced on the shoulder, the extractor has to "hold it in place," which puts a great deal of extra stress on the extractor. This extra stress could lead to accelerated wear, early part failure, and possibly even catastrophic part failure. No one wants small bits of extractor shaped steel flying at high velocity at random directions while on the firing line.
Third, the .380ACP has slightly different extractor groove dimensions and a lower pressure, which leads to lower energy. That energy is needed to force the slide back in order to extract the case from the chamber; a case which has different extractor dimensions, remember. While not especially common, this could lead to a feed failure condition known as "failure to extract" and would most likely result in a stove-pipe condition in which the case doesn't fully eject from the action and "jams" in, sticking out at an angle resembling a stove pipe from a wood burning stove.
["Stove Pipe" failure to eject]
Fourth, most semi-automatic firearms depend on energy imparted to the slide to drive it rearward with enough force for the front of the breech to travel behind the base of the topmost cartridge in the magazine. It must fight against the strength of the recoil spring during this travel. The recoil spring then drives the slide forward again, stripping the topmost cartridge from the magazine and feeding it up the ramp and into the chamber. The strength of the recoil spring is often finely tuned by the gun's designers to fall within the expected energy range of the cartridge the gun is designed to shoot. Because the .380ACP has less energy than the 9x19 Luger, the energy generated may not always be enough to reliably drive the slide rearward with enough force for the breech to travel behind the topmost cartridge. This would cause a feed error in which the slide closes normally but a round is not fed into the chamber.
[Is there a cartridge in the chamber of this gun?]
Finally, because the .380ACP has a shorter over all length, it will fit more loosely in the magazine. The interface between the magazine and the chamber, often a feed "ramp," along with the feed "lips" of the magazine, is often finely tuned to the over-all length range and bullet profile of the cartridge it was designed to shoot. The typical bullet profile of a .380ACP is often more rounded or domed, and obviously the over-all length is shorter, than the 9x19 Luger. This could lead to feed errors such as a live-cartridge stove-pipe or the live cartridge jamming in the chamber at a peculiar angle and failing to properly chamber.
[.380ACP "Stove Pipe" failure to feed]
[.380ACP "Chamber Jam" failure to feed]
Are any of these ills guaranteed to afflict your gun should you press the trigger on a .380ACP cartridge chambered in your 9x19 Luger gun? Of course not. Of the feed errors, my experience is that the most likely is a failure to strip the topmost round and feed it into the chamber. But all of them are a possibility. Will your gun "spontaneously disassemble" if you press the trigger on a .380ACP cartridge chambered in your 9x19 Luger gun? Probably not. But why take the chance? Why would you want to replace the most common and least expensive center-fire pistol cartridge with a more expensive, less available, .380ACP and risk accelerated wear, parts breakage, or, at worst, damage to your gun? The only thing I can think of is if the hordes of Nazi created zombies are beating down your door, you're out of 9mm ammo, and all you have left is that box of .380ACP (and for some reason you can't find the gun you bought it for). Do what the National Rifle Association, SAAMI, and your gun's manufacturer recommends and only use ammunition it was designed for. Unless your gun is designed to chamber and shoot the .380ACP cartridge, don't stick it in your gun.