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 has a well-earned reputation among reloaders as both popular and genuinely temperamental. Introduced in 1990 as a law enforcement round built to approximate the FBI’s downloaded 10mm Auto performance in a mid-size autoloader case, it packs real pressure into a short, wide case that does not forgive the errors a milder cartridge would simply absorb. If you have already built a first batch using our starter guide, this article covers the specific failure points that catch reloaders once they move past the basics and start scaling up.
Whether you are loading for a duty pistol, a competition gun, or range practice, the mistakes below account for the overwhelming majority of problems this cartridge actually produces, from the infamous Glock bulge to over-pressure signs that get missed until it is too late.
Case Bulging from Unsupported Chambers
The Glock bulge is the most notorious issue in 40 S&W reloading, and if you handle mixed range brass, you will run into it eventually. A number of pistol designs, Glock chief among them, use a partially unsupported chamber that lets the case wall near the feed ramp expand outward during firing. After one or two firings, a visible ring or bulge shows up roughly a quarter inch above the case head. Reload that brass without addressing the bulge and you risk chambering problems, case head separation, or a genuinely dangerous failure.
Not every firearm creates this problem equally. Fully supported chambers, the SIG P229 and most 1911-pattern 40s among them, produce far less case expansion. If you are working with brass of unknown origin, assume the worst and inspect every case. A case gauge is essential here: drop each sized case in and watch for anything that hangs up rather than falling freely to the bottom.
Preventing and fixing the bulge:
- Run brass through a small base die or a dedicated bulge buster die, which sizes further down toward the case head than a standard die
- Add this as a distinct step before your normal sizing die, not a substitute for it
- Feel case bodies with your fingers; you can often catch a bulge by touch before you see it
- If you shoot your own reloads exclusively in a fully supported chamber, this problem largely disappears
- Treat any brass from an unknown source as potentially bulged until proven otherwise
Over-Pressure Risk with Fast-Burning Powders
40 S&W runs 35,000 psi maximum average pressure, high for a case this size, and that combination makes it genuinely sensitive to charge variation. Fast-burning powders, Titegroup, Bullseye, or HP-38 among them, stay popular here because they meter cleanly and leave a visible fill level, but they are also the least forgiving of a charging mistake.
A single extra grain of a fast powder can spike pressure into dangerous territory immediately. The deeper problem is that these powders occupy so little case volume that a double charge can still fit without an obvious overflow, unlike a slower powder that would spill visibly. A double charge of Titegroup can look like an ordinary, correctly filled case to a casual glance.
Safer powder selection: consider a medium-burn-rate option, Power Pistol or Longshot among them, for more forgiving charge behavior. These fill more case volume, which makes a double charge genuinely obvious, at the cost of a bit more muzzle flash and, in some cases, a magnum primer requirement. Whatever you choose, consult multiple current manuals, start at the listed minimum, and work up slowly while watching for pressure signs. If you stay with a fast powder, build a strict visual-check routine: after charging each case, look down into the loading block under good light and confirm every fill level matches.
Incorrect OAL and the Feeding Problems It Causes
Overall length affects both feeding reliability and chamber pressure directly. SAAMI’s maximum is 1.135 inches, but plenty of pistols actually feed best somewhere between 1.120 and 1.130. Seat a bullet too long and it can contact the rifling before the case fully chambers, spiking pressure hard; seat it too short and you have reduced case volume, which raises pressure on its own.
Bullet profile matters as much as weight here. A 180-grain flat-nose bullet needs a shorter seating depth than a 180-grain round-nose to feed reliably, which is part of why truncated cone profiles are so common in this cartridge specifically; that shape feeds consistently across a wide range of pistols while keeping pressure in check. Build dummy rounds at your intended OAL, cycle them by hand through your actual magazine and chamber, and adjust in 0.005-inch steps if anything hangs up. A bullet comparator, rather than a straight tip-to-base caliper measurement, gives you a more consistent number across a batch, since bullet tip variation alone can throw off a measurement by several thousandths.
The Double-Charge Danger
Double-charging, accidentally dumping two powder charges into one case, is the scenario every reloader dreads, and 40 S&W is particularly exposed to it. The case is wide enough, and fast powders compact enough, that a doubled charge often still fits without an obvious sign, and the result is a pressure spike capable of destroying a firearm and injuring the shooter.
Progressive presses raise this risk further, since you are not inspecting each charged case individually before a bullet goes on. A moment of distraction, a missed station, or a case that sticks can leave one case charged twice and another charged not at all. The wide case mouth also makes a quick visual check less reliable here than it is with a narrower cartridge like 9mm.
Prevention that actually works:
- Charge cases in a loading block and inspect every single one before seating, without exception
- On a progressive, use a powder check die that stops the press on an out-of-range charge
- Favor a powder that fills at least 40-50 percent of case volume where your data allows it
- Keep strong, direct light over the press
- Never reload while tired, distracted, or interrupted mid-session
- Count your charges; if you have thrown 50 but only have 48 cases, stop and find the discrepancy before continuing
Improperly Resized Brass
Resizing matters more in 40 S&W than in most pistol cartridges, since the high pressure and partially unsupported chambers many pistols use both stretch and expand brass more than 9mm or 45 ACP ever will. Under-resize a case and it won’t chamber reliably. Over-work it and you invite case head separation.
Standard full-length dies often don’t size 40 S&W brass far enough down the body, especially brass fired in a Glock or similar chamber. The result is a case that passes a casual look but hangs up chambering or fails a case gauge check, which is a reliability issue at minimum and a real safety concern if a partially chambered round fires out of battery.
Getting resizing right:
- Run brass through a small base or bulge buster die before your standard sizing die
- Adjust the sizing die to contact the shell holder with moderate cam-over, full sizing without over-working the brass
- Lubricate properly; dry sizing causes stuck cases and inconsistent results
- Gauge-check every resized case; it should drop in freely and sit flush
- If results stay inconsistent, check die adjustment and consider retiring brass that has simply been fired too many times
40 S&W brass typically holds up for 3 to 5 loadings before it should be retired, fewer if it was fired in an unsupported chamber.
Mixing Up 40 S&W and 10mm Auto Brass
This one deserves its own section because the two cases look close enough to fool a distracted reloader, and the consequences of getting it wrong are real. 10mm Auto is 0.142 inches longer than 40 S&W, 0.992 inches against 0.850, not the roughly 0.12-inch gap sometimes quoted. That difference is not interchangeable in either direction; loading 40 S&W data or bullets into 10mm brass, or the reverse, creates excessive headspace and genuinely dangerous pressure variation. The primer type differs too, and it’s worth knowing specifically: 40 S&W uses a standard small pistol primer, while its 10mm Auto parent case uses a large pistol primer. Grab the wrong primer box off a shared shelf and neither substitution ends well. Verify headstamp before you load a single case, every time you’re working with brass from a mixed source.
The Rest of the List
Beyond the major issues above, a handful of smaller mistakes account for most of what’s left:
- Using 9mm dies by mistake. The size similarity has led more than one reloader to grab the wrong set off a crowded bench.
- Ignoring case head separation signs. A shiny ring near the case head signals imminent failure; retire that case immediately.
- Skipping visual inspection. Every case needs a look for cracks, especially around the mouth and web, before it goes back into rotation.
- Not trimming cases. Rare in pistol reloading generally, but 40 S&W brass can stretch enough over its service life to need an occasional trim.
- Mixing headstamps at or near maximum charges. Different manufacturers run different internal case capacities; work up loads separately if you’re pushing toward the top of the range.
- Reloading worn-out brass. A high-pressure round like this one is unforgiving of brass that has already given you what it has to give.
- Neglecting primer pockets. Built-up residue prevents proper seating and invites misfires.
- Over-crimping. Excessive crimp deforms the case mouth and can cause chambering problems of its own.
- Reloading any visibly cracked case. No exceptions; it goes in the scrap bucket.
Safe Reloading Checklist
- Inspect all brass for bulges, cracks, and case head separation signs
- Confirm you’re actually holding 40 S&W brass, not 10mm Auto or 9mm
- Run cases through appropriate resizing dies, small base or bulge buster where needed
- Gauge-check every resized case
- Verify powder type and charge weight against current manual data
- Visually inspect powder level in every charged case
- Measure OAL on your first round and periodically through the session
- Give completed rounds a final visual pass
- Test-chamber several rounds in your actual firearm before range day
- Record your load data for next time
| Pressure Sign | What to Look For | Action Required |
|---|---|---|
| Flattened primers | Primers flush with the case head, or cratered | Reduce charge immediately |
| Difficult extraction | Cases stick or need real force to extract | Stop shooting, reduce charge |
| Case head expansion | A shiny ring or bulge just above the extractor groove | Discard brass, reduce charge |
| Blown primers | Primer falls out or shows gas leakage | Stop immediately; major over-pressure |
| Case splits | Cracks in the body or at the mouth | Discard brass, check all components |
Frequently Asked Questions
What does an over-pressure 40 S&W round actually feel like? Sharper, snappier recoil rather than the normal push, often with a louder report and more visible muzzle flash. If extraction gets difficult or the slide feels sluggish cycling, stop immediately and inspect your brass for the signs above.
Can I reload brass that was fired in a Glock? Yes, but address the bulge first. Run it through a bulge buster or small base die, then confirm with a case gauge. Expect a shorter useful life than brass fired in a fully supported chamber, since it’s already been worked harder to bring it back into spec.
How many times can I reload 40 S&W brass? Most brass handles 3 to 5 loadings in a supported chamber, fewer from a Glock or similar design. Inspect after every firing and retire anything showing cracks, excessive stretching, or head separation signs.
Why won’t my reloads chamber? Usually one of three things: insufficient resizing, especially the case bulge; OAL that’s too long; or over-crimping that has deformed the mouth. Run a case gauge check and confirm OAL against published data for your specific bullet.
What powder is easiest for a reloader still building confidence? A medium-burn option, Power Pistol, Longshot, or Universal among them, is more forgiving than a fast burner. It fills more case volume, which makes a double charge obvious at a glance, and it’s less sensitive to small charge variations while still delivering solid performance.
Quick Takeaways
- A case gauge check on every resized case is non-negotiable with this cartridge
- Small base or bulge buster dies are essential for brass fired in Glocks and similar designs
- Visual inspection of every powder charge is what actually prevents a double-charge disaster
- OAL has to match your specific bullet profile, verified in your actual firearm
- Retire brass after 3 to 5 loadings, or the moment you see a pressure sign
- Never mix 40 S&W and 10mm Auto brass or primers; verify headstamp every time
40 S&W rewards a reloader who respects its pressure and its case geometry, and it punishes shortcuts more directly than most cartridges its size. Case gauge every round, inspect every charge, and don’t hesitate to scrap a questionable case or bullet; the cost of doing that is nothing next to the cost of a damaged gun. Build these habits and this cartridge performs exactly as well as factory ammunition, with the cost savings and customization reloading actually delivers. From here, our guides to best bullets for 40 S&W and best powders for 40 S&W go deeper on the component side, and our overpressure protocol covers exactly what to do if a load has already gone wrong.
Related reading: Best Powders for 40 S&W Handloading.
