The 40 S&W cartridge holds a unique place in American firearms history as the law enforcement round that dominated policing for two decades. Born from a real-world need for better stopping power in a manageable package, it represents a careful balance between the 9mm and the full-power 10mm Auto. Understanding its technical specifications and design quirks is essential for safe reloading and reliable performance.
This guide covers the cartridge’s development history, official SAAMI specifications, case design characteristics, and the critical safety considerations every reloader needs to know. Whether you’re loading for duty use, personal defense, or competition, knowing these fundamentals will help you produce safe, accurate ammunition while avoiding the common pitfalls that plague this high-pressure cartridge.
How the 40 S&W Was Born in the 1990s
The 40 S&W emerged in 1990 as a direct response to the infamous 1986 FBI Miami shootout. That gunfight exposed serious deficiencies in the 9mm’s terminal performance, prompting the FBI to search for a more effective cartridge. The Bureau initially adopted the 10mm Auto, but its full-power loads proved too snappy for many agents, leading to accuracy problems and excessive wear on firearms.
Smith & Wesson and Winchester collaborated to create a solution by downloading the 10mm case to create what many call the “40 Short & Weak” – though that nickname undersells its effectiveness. They shortened the 10mm case from 0.992 inches to 0.850 inches and loaded it to pressures that delivered 10mm-lite performance in a package that fit standard 9mm-frame pistols. The result was immediate adoption by law enforcement agencies across North America, making it the dominant police cartridge from the mid-1990s through the 2010s.
SAAMI Specs
The SAAMI maximum average pressure (MAP) for 40 S&W is 35,000 psi, which places it in the high-pressure handgun category alongside cartridges like 357 SIG. This is significantly higher than 45 ACP (21,000 psi) and even exceeds standard-pressure 9mm (35,000 psi) despite the 40’s larger case volume. That pressure level is what allows the cartridge to push a heavier bullet to similar velocities as 9mm +P loads.
Here are the key SAAMI specifications every reloader should memorize:
| Specification | Measurement |
|---|---|
| Case length | 0.850 inches |
| Overall length (max) | 1.135 inches |
| Case head diameter | 0.424 inches |
| Bullet diameter | 0.400 inches |
| Max pressure (MAP) | 35,000 psi |
The case head diameter of 0.424 inches is identical to 10mm Auto, which makes sense given the 40’s parentage. The relatively short overall length of 1.135 inches allows the cartridge to function in compact pistol frames, but it also creates some unique reloading challenges we’ll address in later sections.
Understanding the 40 S&W Case Design
The 40 S&W features a slight taper from case mouth to base rather than being truly straight-walled. This taper aids in reliable feeding and extraction, but it also means the case doesn’t provide as much bullet grip over its entire length as a straight-walled design would. The case uses a small pistol primer, and the primer pocket depth must be carefully controlled during reloading to prevent primer protrusion.
One often-misunderstood aspect is the case’s semi-rimmed design, though the rim is barely perceptible. The extractor groove is cut into a case head that’s only marginally larger than the case body, creating less material at the head than you’d find in a truly rimless design like 45 ACP. This reduced support area around the case head becomes critically important when combined with the cartridge’s high operating pressure.
The case wall thickness varies along the length of the case, with thinner walls near the mouth and thicker walls toward the base. This graduated thickness is necessary to handle the pressure curve during firing, but it also means cases can behave differently during resizing depending on how much they’ve been worked. Brass hardening occurs faster in 40 S&W cases than in lower-pressure cartridges, making annealing more important for extended case life.
High Pressure Risks and Case Bulge Issues
The combination of 35,000 psi pressure and relatively unsupported case heads in many 40 S&W firearms creates the cartridge’s most notorious problem: case bulging near the base. Glock pistols, in particular, use generous chamber dimensions and minimal case support to ensure reliable feeding under adverse conditions. This design philosophy works perfectly for factory ammunition but creates challenges for reloaders who want to resize fired brass.
When a 40 S&W cartridge fires in a chamber with limited support, the case head expands to fill the available space. The brass near the web area (just above the extractor groove) can bulge beyond the dimensions that a standard resizing die will address. If you simply resize the case mouth and body without addressing this base bulge, the round may not chamber properly or could create dangerous pressure spikes. Many reloaders have experienced rounds that pass a case gauge but won’t chamber in their pistol because of this invisible bulge.
A bulge buster die or case gauge becomes nearly essential equipment for anyone reloading 40 S&W in volume. These specialized tools resize the lower portion of the case that standard dies miss. Some reloaders use a Lee Factory Crimp Die (FCD) for this purpose, while others invest in dedicated bulge-busting equipment. Regardless of your method, every case should be checked with a proper case gauge before loading.
Quick Checklist: Inspecting 40 S&W Brass
- Check each case with a dedicated 40 S&W case gauge before and after resizing
- Inspect for bulges near the case head by rolling cases on a flat surface
- Look for cracks starting at the case mouth or just above the extractor groove
- Discard any cases with loose primer pockets (primers seat with little resistance)
- Verify case length stays within 0.845-0.850 inches after trimming
- Check for case head separation by inserting a bent paper clip inside cases
- Examine the case mouth for splits or deformation from previous crimping
- Ensure primer pockets are clean and free of debris
Common Mistakes
The most frequent error is ignoring base bulges and assuming a standard resizing die handles everything. Many reloaders discover this problem only after producing hundreds of 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 rounds chamber with difficulty or require excessive force, you’ve likely got base bulging that needs correction.
Another critical mistake is pushing maximum charges without understanding your specific firearm’s chamber dimensions. Published load data represents safe pressures in SAAMI-spec test barrels, but your pistol’s chamber may provide less case support than the test equipment. Start at the low end of published data and work up carefully, watching for pressure signs like flattened primers, difficult extraction, or excessive case head expansion. The 40 S&W operates close to its pressure limits even with mid-range loads.
Bullet setback causes dangerous pressure spikes in any cartridge, but it’s especially hazardous in the already-high-pressure 40 S&W. When a bullet gets pushed deeper into the case (from repeated chambering or excessive crimp), the reduced case volume causes pressure to skyrocket. Some reloaders have reported catastrophic failures from bullets that moved just 0.050 inches deeper than intended. Always measure overall length on carry ammunition periodically and retire any rounds showing setback.
Here are additional mistakes to avoid:
- Using 9mm dies instead of 40 S&W-specific tooling
- Failing to sort brass by headstamp (different manufacturers have varying case capacities)
- Over-crimping, which can cause case mouth splits and bullet deformation
- Mixing large and small primer brass without sorting (some manufacturers use large primers)
- Ignoring published overall length specifications to gain powder capacity
- Reloading Glock-fired brass beyond 3-4 cycles without annealing
FAQ:
Is 40 S&W just a shortened 10mm Auto?
Yes, the 40 S&W is essentially a shortened 10mm Auto case loaded to moderate pressures. The 10mm case measures 0.992 inches while the 40 S&W measures 0.850 inches. Both share the same 0.424-inch case head diameter and use 0.400-inch diameter bullets. You cannot, however, simply trim 10mm brass to make 40 S&W cases – the case walls would be too thick and the internal dimensions wouldn’t match SAAMI specifications.
Can I use 10mm load data for 40 S&W?
Absolutely not. While the cartridges share lineage, they operate at different pressures with different powder capacities. The 40 S&W’s shorter case means reduced volume, which creates higher pressures with the same powder charge. Always use 40 S&W-specific load data from reputable sources like powder manufacturers’ manuals. Cross-referencing data between cartridges is a recipe for dangerous pressure excursions.
Why do my resized cases still fail the case gauge?
The most common cause is unsupported case bulging near the base that your standard resizing die doesn’t reach. The die sizes the upper portion of the case but leaves the bulge at the web area. You need either a bulge buster die, a Lee Factory Crimp Die used as a post-sizing step, or a carbide sizing die specifically designed for Glock-fired brass. Some reloaders also use an undersized sizing die (like a 40 S&W die set to 9mm dimensions) to address stubborn bulges.
How many times can I reload 40 S&W brass?
That depends heavily on the firearm that fired it and how you process the brass. Glock-fired brass typically lasts 3-5 loadings before primer pockets loosen or case head issues develop. Brass from pistols with better-supported chambers (like SIG P229 or Beretta 96) can last 8-10 cycles or more. Always inspect each case before loading and retire any brass showing signs of fatigue, regardless of cycle count.
Is 40 S&W dying out as a cartridge?
While many law enforcement agencies have returned to 9mm due to improved bullet technology and higher capacity, the 40 S&W remains popular for personal defense and competition shooting. The used market is flooded with excellent 40 S&W pistols at attractive prices, and ammunition remains widely available. For reloaders, the cartridge offers an interesting technical challenge and excellent ballistic performance when loaded properly.
What’s the best powder for 40 S&W reloading?
Popular choices include Power Pistol, Unique, WSF, and Titegroup for lighter target loads, while Blue Dot, Longshot, and CFE Pistol work well for full-power defensive loads. The “best” powder depends on your bullet weight, desired velocity, and intended use. Always consult multiple load manuals and start at the minimum listed charge. The 40 S&W’s high pressure means there’s less margin for error than with lower-pressure cartridges.
The 40 S&W represents an interesting chapter in American firearms development and remains a capable cartridge for defense and competition use. Its high operating pressure and unique case support challenges make it more demanding to reload than cartridges like 45 ACP or even 9mm, but understanding these characteristics allows you to produce reliable, accurate ammunition safely. The key is respecting the cartridge’s pressure limits and addressing the case bulging issues that come with fired brass.
If you’re new to reloading 40 S&W, invest in proper case gauges and bulge-reducing equipment before you start producing ammunition in volume. Take the time to inspect each case thoroughly and work up loads carefully from minimum charges. The cartridge’s reputation for being “difficult” to reload comes primarily from reloaders who skipped these fundamental steps. With proper technique and attention to detail, the 40 S&W can provide years of reliable service and significant savings over factory ammunition.
