Understanding the 40 S&W: History and Specifications

Published: February 2026 | Last updated: September 2026

Disclaimer: This article is for informational and educational purposes only. Reloading involves inherent risks. Always consult current manufacturer load data, follow established safety protocols, and work up loads incrementally. Neither the author nor MyReloading.com assumes liability for the use of the information presented here.

40 S&W holds a genuinely unique place in American firearms history as the law enforcement cartridge that dominated policing for two decades. Born from a real-world need for better stopping power in a package that still fit a standard 9mm-size frame, it represents a careful compromise between 9mm and the full-power 10mm Auto that preceded it. Understanding its technical specifications and design quirks matters directly for safe, reliable reloading, not as background trivia.

This is the hub page for everything MyReloading.com covers on this cartridge. It walks through the development history, the official SAAMI specifications, the case design that makes this round more demanding to reload than most, and the safety fundamentals every handloader needs before touching a press. From here, our narrower guides cover the specific work: your first handload, self-defense loads, carbine builds, bullet and powder selection, and how this cartridge stacks up against 9mm.

How 40 S&W Was Born in the 1990s

40 S&W emerged in 1990 as a direct response to the 1986 FBI Miami shootout, a gunfight that exposed real deficiencies in the terminal performance of the 9mm and 38 Special ammunition federal agents carried at the time. In the aftermath, the FBI began testing more powerful alternatives and initially adopted the 10mm Auto, a cartridge Jeff Cooper had helped develop at Gunsite over a decade earlier. Full-power 10mm loads proved too snappy for a lot of agents to control well, leading to accuracy problems and accelerated wear on the pistols chambered for it, so the Bureau moved to a reduced-velocity, downloaded 10mm variant instead, one that still left considerable unused airspace in that long case.

Smith & Wesson and Winchester saw an opportunity in that airspace. Rather than continue loading a full-length case to reduced velocity, they shortened the 10mm case from 0.992 inches down to 0.850 and loaded it to a pressure that delivered comparable performance in a case genuinely sized to fit standard 9mm-frame pistols. The new cartridge, and the Smith & Wesson Model 4006 built to fire it, both debuted on January 17, 1990, though production pistols took several more months to actually reach dealers. Some early skeptics dismissed it as “40 Short & Weak,” a jab that undersold what the round actually delivered in the field. Law enforcement adoption followed almost immediately and made 40 S&W the dominant American police cartridge from the mid-1990s well into the 2010s. Our 40 S&W vs 9mm Luger comparison covers exactly how that ballistic tradeoff plays out at the bench today.

SAAMI Specs: Dimensions and Maximum Pressure

The Sporting Arms and Ammunition Manufacturers’ Institute (SAAMI) sets 40 S&W’s maximum average pressure at 35,000 psi, placing it in the high-pressure handgun category alongside 357 SIG and matching standard-pressure 9mm despite 40 S&W’s larger overall case volume. That pressure figure is significantly above 45 ACP’s 21,000 psi ceiling, and it’s exactly what lets this cartridge push a heavier bullet to velocities that rival 9mm +P performance.

Key SAAMI dimensions:

Specification	Measurement
Case length	0.850″
Overall length (max)	1.135″
Case head diameter	0.424″
Bullet diameter	0.400″
Max pressure (MAP)	35,000 psi
Primer type	Small pistol

The case head diameter of 0.424 inches is identical to 10mm Auto, which makes sense given 40 S&W’s direct lineage from that cartridge. The relatively short 1.135-inch maximum overall length is what lets it function reliably in compact pistol frames, and it’s also the source of some of the reloading challenges covered later in this guide.

Understanding the 40 S&W Case Design

The case carries a slight taper from mouth to base rather than being a truly straight-walled design, which aids feeding and extraction but also means it grips a seated bullet somewhat differently across its length than a purely straight case would. It uses a small pistol primer, and primer pocket depth needs careful, consistent control during reloading to avoid any protrusion.

One point worth correcting directly, since it shows up wrong often enough to matter: 40 S&W is a rimless cartridge, not semi-rimmed. Its rim diameter and case head diameter are identical, both 0.424 inches, which is the defining characteristic of a rimless design; a semi-rimmed case would show a rim measurably larger than the body immediately ahead of it. The extractor groove is cut into a case head that provides genuinely less surrounding material than you’d find on a cartridge like 45 ACP, and that reduced support area is exactly what makes the cartridge’s high operating pressure a real consideration rather than an abstract spec sheet number.

Case wall thickness varies along the case length, thinner near the mouth and thicker toward the base, a graduated design necessary to handle the pressure curve during firing. That variation also means a case can behave differently during resizing depending on how much prior working it’s already seen. Brass hardens faster in 40 S&W than it does in lower-pressure cartridges, which is part of why annealing matters more here for anyone trying to extend case life meaningfully.

High Pressure Risks and the Case Bulge Problem

The combination of 35,000 psi and the relatively unsupported chambers found in a lot of 40 S&W pistols produces this cartridge’s single most notorious reloading problem: case bulging near the base. Glock pistols specifically use generous chamber dimensions and minimal case support, a design choice that serves feeding reliability under adverse conditions well but creates real headaches for anyone trying to resize that fired brass.

When a round fires in an unsupported chamber, the case head expands to fill the available space, and the brass near the web, just above the extractor groove, can bulge past what a standard sizing die is built to correct. Resize only the case mouth and body without addressing that base bulge, and the round may not chamber reliably or, worse, may create a genuine pressure problem. Plenty of reloaders have discovered rounds that pass a casual case gauge check but still won’t chamber cleanly in their actual pistol, because of exactly this invisible bulge sitting lower on the case than most gauges check.

A bulge buster die or a dedicated case gauge is close to essential equipment for anyone reloading this cartridge in real volume. These tools address the lower portion of the case that a standard die misses entirely. Some reloaders press a Lee Factory Crimp Die into service for this same purpose; others invest in dedicated bulge-correcting equipment. Whichever method you use, every case deserves a genuine gauge check before it goes anywhere near a loaded magazine.

Quick checklist for inspecting 40 S&W brass:

Check every case with a dedicated case gauge, both before and after resizing
Roll cases on a flat surface to feel for a bulge near the head
Look for cracks starting at the mouth or just above the extractor groove
Discard any case with a loose primer pocket, one that seats with little resistance
Verify trimmed case length stays within 0.845-0.850 inches
Check for case head separation with a bent paper clip run inside the case
Examine the mouth for splits or deformation left from a previous crimp
Confirm primer pockets are clean and free of debris before priming

The Mistakes That Show Up Most Often

The most frequent error is assuming a standard resizing die handles the whole case, base bulge included. Plenty of reloaders discover this only after producing a few hundred rounds that won’t chamber reliably. Always resize fired 40 S&W brass with equipment that addresses the entire case length, not just the upper portion; if a round chambers with real resistance or needs extra force, you’ve very likely got base bulging that still needs correcting.

A second critical mistake is pushing toward maximum charges without accounting for your specific firearm’s actual chamber support. Published data represents safe pressure in a SAAMI-spec test barrel, and your particular pistol’s chamber may offer meaningfully less case support than that test equipment does. Start at the low end of published data and work up carefully, watching for pressure signs, flattened primers, difficult extraction, or excessive case head expansion, since this cartridge runs close to its limits even at mid-range charges.

Bullet setback deserves its own warning here specifically because the consequences arrive fast in an already-high-pressure case. When a bullet gets pushed deeper into the case, from repeated chambering or from a crimp that isn’t doing its job, the resulting drop in case volume sends pressure up sharply. Reports of real failures tied to a bullet that moved just 0.050 inches deeper than intended are not exaggeration; measure overall length on carry ammunition periodically and retire anything showing setback rather than trusting it. Our common 40 S&W mistakes guide covers this and the rest of the cartridge’s specific failure points in real depth.

Additional mistakes worth knowing:

Reaching for 9mm dies instead of tooling built specifically for this cartridge
Skipping headstamp sorting; different manufacturers run genuinely different case capacities
Over-crimping, which can split a case mouth and deform the bullet underneath it
Mixing large and small primer brass without sorting, since some manufacturers’ 10mm-adjacent brass uses large primers where 40 S&W uses small
Ignoring published overall length specs to squeeze out a little extra powder capacity
Reloading Glock-fired brass past 3-4 cycles without annealing to compensate for the extra working it’s seen

Choosing Components: Powder, Bullets, Primers

Powder selection for 40 S&W spans a real range depending on purpose: fast options like Titegroup and HP-38, sold under the Winchester name as 231, for economical practice ammunition; medium-burn choices like Unique and Power Pistol for defensive loads and general-purpose work; and medium-slow powders like Blue Dot and Longshot when you’re chasing genuine velocity with a heavier bullet. Our best powders for 40 S&W guide breaks all of these down with real charge and velocity data, including a documented disagreement between two major manuals worth knowing about before you trust either one blindly.

Bullet weight comes down largely to purpose: 165-grain for flatter trajectory and reduced recoil, useful in competition and fast follow-up work, or 180-grain for the momentum and consistent expansion that make it the default choice for defensive loading. The Sierra Sports Master is one proven option worth knowing by name. Our best bullets for 40 S&W guide covers construction, weight, and the specific bullets with real gel-test data behind them.

Primers are straightforward: standard small pistol primers, CCI 500, Federal 100, or Winchester WSP, cover the overwhelming majority of loads here, with a magnum-rated primer reserved for the rare recipe that specifically calls for one, typically paired with a slower powder in cold conditions. For case prep and die setup fundamentals that apply to any 40 S&W build, our case prep essentials guide and our walkthroughs on tuning a sizing die and tuning a bullet seating die round out the bench basics this cartridge leans on hardest. A digital powder scale and a set of pistol dies built for this cartridge specifically round out a solid bench setup.

Platforms: Where 40 S&W Actually Gets Fired

Glock’s 22 and 23 remain the platforms most closely associated with this cartridge, thanks largely to Glock beating Smith & Wesson’s own 4006 to store shelves by about a week when the caliber launched. The Smith & Wesson M&P line, SIG P226/229, and Beretta 96 all carry real duty and civilian followings too, and the chamber support differences between these designs directly affect how long your brass actually lasts, a fully supported chamber like the SIG P229’s meaningfully outlasting Glock-fired brass in most reloaders’ experience.

Beyond the pistol, 40 S&W runs in a genuine pistol-caliber carbine niche, where a longer barrel extracts real velocity, typically 150 to 250 fps over pistol figures, and opens the door to slower powders a 4-inch barrel would waste. Our 40 S&W in a pistol-caliber carbine guide covers everything specific to that build, and our 9mm in a pistol-caliber carbine guide is worth a look if you’re weighing that platform against the 9mm alternative.

Getting Started: First Handloads and Defensive Loads

If you haven’t loaded this cartridge before, our first 40 S&W handload guide walks through a genuinely conservative starter recipe along with the case-gauge, OAL, and crimp fundamentals this hub only summarizes. If you’re building specifically toward a carry load, our 40 S&W self-defense guide covers the real expansion-velocity window for modern bonded bullets and the reliability testing that separates a genuinely trustworthy carry load from one that just happens to group well at the range, and our overpressure protocol is worth reading before you push any load toward its published maximum.

Interesting Facts, Trivia, and Unusual Uses

A few things about this cartridge worth knowing, even if none of them change how you load a single round.

Glock beat Smith & Wesson to market with S&W’s own cartridge. Despite co-developing 40 S&W with Winchester, Smith & Wesson didn’t actually get the first 40-caliber pistol onto dealer shelves. Glock’s 22 and 23 were announced a week before the Model 4006 and reached stores first, an advantage built on Glock having already engineered a 10mm pistol, the Glock 20, a short time earlier; adapting that design to the shorter 40 S&W case was a comparatively small step for them, while S&W’s dedicated 4006 needed more development time to reach production.

The FBI adopted Glock’s gun, not Smith & Wesson’s, for the cartridge S&W helped invent. After testing both the 4006 and the Glock 22 head to head, the Bureau formally adopted the Glock 22 as its duty pistol in May 1997, seven years after the cartridge itself launched. The company that helped create 40 S&W spent years watching a competitor’s pistol become the caliber’s defining platform in federal service.

The cartridge’s real intellectual roots predate the FBI shootout by two decades. A wildcat lineage traces back to 1965, when gun writer George Nonte built an experimental cartridge off the then-new 41 Magnum case, shortening it and rebating the rim for semi-auto use. Guns & Ammo associate editor Whit Collins picked up that thread in the following years, sketching a 40-caliber concept based on the 30 Remington rifle case loaded with a 180-grain 38-40 Winchester bullet, work that circulated in wildcatting circles well before Jeff Cooper’s 10mm Auto or the FBI’s own downloaded-10mm experiment ever entered the picture.

“40 Short & Weak” was meant as an insult and became a badge of honor. 10mm loyalists coined the nickname to needle the newer, milder cartridge for backing off the full-power round’s velocity. Two decades of dominant law enforcement adoption made the jab feel increasingly beside the point, even as some of the same 10mm advocates eventually got their comeback when a number of agencies, the FBI included, moved back toward 9mm in the 2010s, citing modern bullet technology closing the performance gap along with the training and capacity advantages a milder cartridge offers.

Reloaded 40 S&W ammunition has a specific, well-documented failure mode with its own nickname. Case failures tied to the cartridge’s high pressure and unsupported-chamber bulging problem are commonly referred to in the industry as “kaBooms” or “kB!” for short, and most documented incidents trace back to reloaded or remanufactured ammunition loaded at or above SAAMI pressure, or to slightly oversized cases firing just out of battery. It’s a blunt, memorable reminder of exactly why the case-bulge inspection habits covered earlier in this guide matter in practice, not just on paper.

The same bullet diameter connects it directly to its parent cartridge. 40 S&W and 10mm Auto both use 0.400 to 0.401-inch bullets, meaning any bullet built for one works in the other without modification; only the case, powder charge, and resulting pressure differ. That shared bullet diameter is part of why component shelves stocking one caliber’s projectiles are quietly stocking the other’s too.

Frequently Asked Questions

Is 40 S&W just a shortened 10mm Auto? Essentially, yes. 40 S&W is a 10mm Auto case shortened from 0.992 inches down to 0.850 and loaded to a more moderate pressure. Both share the identical 0.424-inch case head diameter and 0.400-inch bullet diameter. You cannot simply trim 10mm brass down to make 40 S&W cases, though; wall thickness and internal dimensions wouldn’t match SAAMI specification even after trimming to length.

Can I use 10mm load data for 40 S&W? Absolutely not. The cartridges share lineage but operate at genuinely different pressures and case capacities, and 40 S&W’s shorter case means less volume, which drives higher pressure from an identical powder charge. Always use data specific to 40 S&W from a reputable current source; cross-referencing between these two cartridges is a direct path to a dangerous pressure excursion.

Why do my resized cases keep failing the case gauge? Almost always unsupported base bulging that a standard die doesn’t reach. The die sizes the upper case fine but leaves the bulge sitting at the web. A bulge buster die, a Lee Factory Crimp Die used as a dedicated post-sizing step, or a carbide die built specifically for Glock-fired brass all address this; pick one and make it a standard part of your process rather than an occasional fix.

How many times can I reload 40 S&W brass? It depends heavily on which firearm fired it and how carefully you process it afterward. Glock-fired brass typically handles 3 to 5 loadings before primer pockets loosen or case head problems develop. Brass from a better-supported chamber, SIG P229 or Beretta 96 among them, can go 8 to 10 cycles or more. Inspect every case regardless of count and retire anything showing real fatigue.

Is 40 S&W a dying cartridge? Plenty of law enforcement agencies have returned to 9mm as bullet technology improved and magazine capacity became a bigger priority, but 40 S&W remains genuinely popular for personal defense and competition shooting. The used market carries excellent pistols at real discounts, and ammunition and components stay widely available. For a reloader, it remains an interesting technical challenge with real ballistic reward when loaded properly.

What’s the best powder for 40 S&W? Depends entirely on purpose. Power Pistol, Unique, and Titegroup suit lighter target loads well; Blue Dot, Longshot, and CFE Pistol handle full-power defensive or hunting work. The powder guide linked above breaks the full field down with actual charge and velocity data rather than a general recommendation.

Quick Takeaways

40 S&W debuted January 17, 1990, developed by Smith & Wesson and Winchester from a shortened 10mm Auto case
SAAMI maximum pressure is 35,000 psi, in the same high-pressure tier as 357 SIG
It is a rimless cartridge, not semi-rimmed; rim diameter and case head diameter are identical at 0.424 inches
Case bulging near the base, especially from Glock-fired brass, is this cartridge’s signature reloading challenge
Never substitute 10mm Auto load data; the shorter case means genuinely different pressure behavior
Inspect every case with a real gauge check, and address base bulging with dedicated equipment, not a standard die alone
Glock-fired brass typically lasts 3-5 loadings; brass from better-supported chambers can go 8-10 or more

40 S&W represents a genuinely interesting chapter in American cartridge development, and it remains a capable, well-supported choice for defense, duty, and competition use today. Its real operating pressure and case support challenges make it more demanding to reload than 9mm or 45 ACP, but understanding those characteristics is what actually lets you produce reliable, accurate ammunition safely rather than finding out the hard way. Respect the pressure ceiling, address case bulging directly rather than hoping a standard die handles it, and work up loads carefully from genuine minimums. Do that, and this cartridge rewards you with exactly the reputation that built its two decades of law enforcement dominance in the first place.