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Bore and groove diameters
Bore and groove diameters
Courtesy of Ronnie Freels (see reuse policy).

The bore diameter of the barrel of a rifled firearm is defined as the diameter of the circle formed by the tops of the lands inside the barrel. This diameter does not include the grooves within the barrel. However, a portion of the mass of a fired bullet is extruded into these grooves. Therefore, the diameter of a fired bullet will approximate the larger groove diameter and will always be greater than the bore diameter of the firearm.

The measured diameter of fired bullets is taken from one groove impression (a high point on a fired bullet) to a groove impression on the opposite side of the bearing surface. If there are an odd number of groove impressions, the measurement is taken from the edges of a pair of impressions. The base of the bearing surface of a bullet is used because it is generally more protected when a bullet impacts with an object. Often the bearing surface endsat the base of the entire bullet, althoughwith boattailed bullets this would not be the case. For bullets that are severely flattened or distorted, it may be possible to measure the circumference and calculate an approximate diameter.

Significant base mutilation
Courtesy of Ronnie Freels (see reuse policy).

Whenever measurements are taken, avoid damaging or obliterating microscopic marks on the groove impressions with measuring instruments such as calipers. Plastic calipers can minimize this potential problem.

In the following situations, a measurement should be taken at an alternate location:

  • Significant base mutilation upon impact, altering the width or apparent caliber
  • Unusual amount of base expansion after firing (e.g., deep hollow-based lead bullets)
  • Unusual base expansion of lead bullets after firing from a short-barreled handgun
Deep hollow-based lead bullets, held by calipers.
Deep hollow-based lead bullets, held by calipers.
Courtesy of Ronnie Freels (see reuse policy).
Courtesy of Ronnie Freels (see reuse policy).


Possible Cartridge Types

In the following chart, the measured diameter of a bullet in inches corresponds to possible cartridge type(s).

Possible Cartridge Types Indicated Based on the Measured Diameter of a Fired Bullet
.202" - .228" 22
.247" - .271" 25 Auto
.302" - .328"

32 Auto
32 H&R Magnum
32 S&W
32 S&W L

.353 " - .373" .357 Mag.
38 Spl.
380 Auto
9mm (various types)
.401" - .410" 40 S&W
41 Rem Mag
10mm Auto
.425" - .431" 44 Rem. Mag.
.447" - .458" 45 Auto
45 Colt
455 Webley/Colt
* assumes little or no deformation that makes measurement difficult

Measured caliber/diameter, as with other physical features, is only one factor to consider. A bullet of a given caliber may be consistent with any number of cartridge types.

One caliber and multiple cartridge types
Courtesy of Jack Dillon (see reuse policy).


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