Overbore Index

Below is a table of calibers that have already had their overbore index calculated.

If you are unable to find your caliber or wish to enter a Wildcat cartridge, scroll to the bottom of this page to calculate it.

Not sure what Overbore means or how it's used? Click here to learn more.

The equation used to calculate the Overbore Index on my site is below.

I did not create the original equation but I also don't recall where I found it, so if you want credit for it then let me know and I'd be happy to note that here.

Bore Area = π × (Bullet Diameter ÷ 2)2
Overbore Index = Case Capacity ÷ Bore Area (Output is rounded and concatenated to the hundredths place)

Overbore Indices Table

Table is sorted by bullet diameter then case capacity, smallest to largest.

Cartridge Name Case Capacity (gr. of water) Bullet Diameter (in.) Overbore Index
HK 4.6 × 30mm 13.4 0.183" 509.46
5.45 × 39mm 27.0 0.220" 710.28
FN 5.7 × 28mm 13.9 0.224" 352.72
5.56 × 45mm (.223 Remington) 28.5 0.224" 723.20
5.8 × 42mm Chinese 32.6 0.236" 745.25
6 × 38mm (6mm ARC) 34.0 0.243" 733.12
6 × 52mm (.243 Winchester) 54.8 0.243" 1181.62
6.5 × 38mm (6.5 Grendel) 35.0 0.264" 639.40
6.5 × 48mm (6.5 Creedmoor) 52.5 0.264" 959.09
6.8 × 43mm (6.8 SPC) 36.9 0.277" 612.32
7.62 × 51mm NATO (.308 Winchester) 52.0 0.308" 697.93
7.62 × 35mm (.300 Blackout) 25.1 0.309" 334.71
7.62 × 39mm 35.6 0.310" 471.67
7.62 × 54mmR 64.2 0.312" 839.72
7.92 × 33mm Kurz 34.3 0.324" 416.02
7.92 × 57mm Mauser (8mm Mauser) 63.1 0.324" 765.33
9 × 19mm Parabellum (9mm Luger) 13.3 0.355" 134.37
9 × 33mmR (.357 Magnum) 26.2 0.357" 261.74
10.57 × 73mm (.416 Rigby) 129.0 0.416" 949.10
11.43 × 23mm (.45 ACP) 27.4 0.452" 170.76
12.7 × 99mm NATO (.50 BMG) 292.8 0.511" 1427.71

All case capacities in this table were retrieved from open source data. In situations where the case capacity was specified as a range, the maximum was used as it provides the most conservative estimate.

Overbore Index Calculator

Overbore Index

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