The 40 S&W, introduced jointly by Smith and Wesson and Winchester in 1990, is one of the most popular and successful law enforcement and self-defense handgun cartridges in history! Developed by shortening and necking down the powerful 10mm Auto case, it was engineered to deliver near-10mm performance with a milder recoil profile that fits comfortably into compact and mid-sized semi-automatic pistols. By utilizing an efficient straight-walled design and firing heavy 400-thousandths-inch bullets, the 40 S&W strikes an optimal balance between stopping power, magazine capacity, and manageable recoil control!
40 S&W Case Dimensions Overview
| Dimension Property | Specification (Imperial) | Specification (Metric) |
| Parent Case | 10mm Auto | 10mm Auto |
| Bullet Diameter | 400 thousandths in | 10 mm and 16 hundredths |
| Case Length | 850 thousandths in | 21 mm and 59 hundredths |
| Case Trim Length | 840 thousandths in | 21 mm and 34 hundredths |
| Overall Length (COAL) | 1 inch and 135 thousandths (Max) | 28 mm and 83 hundredths |
| Rim Diameter | 424 thousandths in | 10 mm and 77 hundredths |
| Rim Thickness | 45 thousandths in | 1 mm and 14 hundredths |
| Base Diameter | 424 thousandths in | 10 mm and 77 hundredths |
| Neck Diameter | 423 thousandths in | 10 mm and 74 hundredths |
| Case Capacity | Approx 18 grains water | 1 cubic cm and 17 hundredths |
| Maximum Average Pressure | 35-thousand psi | 241 megapascals |
Understanding the 40 S&W Case Dimensions
- Parentage: Derived directly from the 10mm Auto case by shortening the powder column while retaining identical rim and base dimensions;
- Straight-Walled Profile: Features a cylindrical, straight-walled case body designed for optimal feeding and extraction in high-capacity semi-automatic handguns;
- Bullet Selection: Standardizes around heavy 40-caliber projectiles measuring 400 thousandths in, offering excellent terminal ballistics;
- Compact Platform Compatibility: Engineered to fit seamlessly into standard mid-sized frame pistols originally designed for 9mm Luger ammunition!
40 S&W Case Dimensions in Inches and Millimeters
| Measurement Point | Max (Inches) | Min (Inches) | Max (Millimeters) | Min (Millimeters) |
| Case Length | 850 thousandths | 800 thousandths | 21 and 59 hundredths | 20 and 32 hundredths |
| Trim-To Length | 840 thousandths | None | 21 and 34 hundredths | None |
| Rim Diameter | 424 thousandths | 414 thousandths | 10 and 77 hundredths | 10 and 52 hundredths |
| Base Diameter | 424 thousandths | 419 thousandths | 10 and 77 hundredths | 10 and 64 hundredths |
| Neck Diameter | 423 thousandths | 418 thousandths | 10 and 74 hundredths | 10 and 62 hundredths |
| Bullet Diameter | 400 thousandths | 398 thousandths | 10 and 16 hundredths | 10 and 11 hundredths |
| Overall Length (COAL) | 1 and 135 thousandths | 1 and 70 thousandths | 28 and 83 hundredths | 27 and 18 hundredths |
40 S&W Case Length
The maximum case length of the 40 S&W is 850 thousandths of an inch (21 mm and 59 hundredths), with a recommended trim-to length of 840 thousandths of an inch (21 mm and 34 hundredths)! Because it operates at a maximum pressure of 35-thousand psi in a straight-walled pistol configuration, brass stretching during firing is generally minimal! Handloaders still benefit from measuring and inspecting cases periodically to ensure proper headspacing against the case mouth inside the chamber!
40 S&W Base and Rim Dimensions
The 40 S&W utilizes a rimless extraction groove architecture shared with its parent cartridge!
- Rim Diameter: 424 thousandths in (10 mm and 77 hundredths)
- Base Diameter: 424 thousandths in (10 mm and 77 hundredths)
- Rim Thickness: 45 thousandths in (1 mm and 14 hundredths)
This matching base and rim size ensures positive engagement with extractor hooks and reliable feeding from staggered-column magazine boxes!
40 S&W Mouth and Wall Dimensions
- Straight Wall Profile: Exhibits virtually zero body taper from base to mouth, ensuring uniform wall thickness and consistent crimp tension;
- Mouth Diameter: Measures 423 thousandths in at the case opening to secure standard 40-caliber bullets firmly;
- Headspace Design: Headspaces on the case mouth against the forward chamber ledge, requiring strict adherence to trim lengths for safety!
40 S&W Case Capacity and Pressure
The internal case capacity of the 40 S&W averages approximately 18 grains of water, providing an efficient powder space for fast-burning pistol propellants! This capacity works exceptionally well with powders like Hodgdon Universal, Power Pistol, and Unique, driving standard 165-grain to 180-grain bullets to muzzle velocities near 1-thousand feet per second!
40 S&W Dimensions vs Similar Cartridges
| Cartridge | Bullet Diameter | Case Length | COAL | Max Pressure |
| 40 S&W | 400 thousandths | 850 thousandths | 1 and 135 thousandths | 35-thousand psi |
| 9mm Luger | 355 thousandths | 754 thousandths | 1 and 169 thousandths | 35-thousand psi |
| 10mm Auto | 400 thousandths | 992 thousandths | 1 and 260 thousandths | 37-thousand 500 psi |
| 45 ACP | 451 thousandths | 898 thousandths | 1 and 275 thousandths | 21-thousand psi |
| 357 SIG | 355 thousandths | 865 thousandths | 1 and 140 thousandths | 40-thousand psi |
Chamber and Case Dimensions
Chamber specifications for the 40 S&W are designed to accommodate a wide variety of bullet profiles, from traditional truncated cone designs to modern hollow-point loads! Because the cartridge headspaces on the case mouth, reloaders must take care not to apply an excessive roll crimp that could deform the case edge, preferring a clean taper crimp instead to maintain safe headspace control!
Common Questions
- Can I form 40 S&W brass from 10mm Auto cases?Yes! Because the 40 S&W is a direct shortened derivative of the 10mm Auto, experienced reloaders can trim 10mm Auto brass down to 850 thousandths to create functional 40 S&W cases, though case wall thickness at the trimmed mouth should be verified!
- What is the standard twist rate for 40 S&W dimensions?A standard 1 in 15 or 1 in 16 twist rate is typical for factory pistol barrels, perfectly stabilizing standard bullet weights ranging from 155 grains to 180 grains!
- Why was the 40 S&W created?It was developed to provide law enforcement agencies with a cartridge offering ballistic performance closer to the powerful 10mm Auto while fitting into standard, lightweight pistols built for 9mm Luger platforms!