Explaining Diameter Measurements Used in Wire, Nail, and Bullet Sizes

Two bullets
The size of a bullet is measured in calibers.
Cliff Volpe/Getty Images

Deciphering the specifics of bullet sizes, wire gauges, and nail diameters offers unexpected insight into the precision engineering behind everyday objects. From the defense industry to household utilities, these measurements have significant implications. Here, we explore these seemingly cryptic numbers and designations and what they mean in practical terms.


Understanding Bullet Size

Bullets come in a variety of different calibers. While it might sound like cryptic bullet terminology, the caliber indicates the bullet diameter. Thus the diameter is the bullet size.

Some common bullet calibers include:


  • .22 (the .22 caliber rifle is popular for hunting and target practice)
  • .25
  • .357
  • .38
  • .44
  • .45

The gauge of shotgun shells is also important. The gauge numbers for a shotgun (12 gauge, 14 gauge, etc.) are the numbers of lead balls — of diameters equal to the inside diameter of the gun — required to total up to one pound of lead.


Explaining Wire Dimateres

­The measurement system for wire is also interesting. AWG stands for "American Wire Gauge" and is a standard in the United States for wire diameters. In a house, you typically find 10, 12 and 14 gauge wire, and electronics typically use 20 gauge wire. Wire gauges run from 000000 gauge, which is ­about half an inch in diameter, down to 40 gauge, which is 0.001 inches. The higher the AWG number, the thinner the wire.

Where did this odd system come from? The modern method for wire drawing is thought to have emerged in Europe in the latter part of the 13th century. This method involves pulling the metal through a series of progressively smaller holes until it reaches the required diameter. The gauge number stems from how many holes the wire is drawn through. And the more holes it's drawn through, the smaller the diameter.


Looking at Nail Sizes

Then there's the common penny designation used with nails. You can buy nails ranging from twopenny to fiftypenny in the United States (represented as 2d or 50d, respectively).

Why is "d" used instead of "p"? This stems from Roman times. The "d" stood for "denarius," a small coin common throughout the Roman empire. The penny designation can be translated into length. It's not clear that anyone knows where the penny designation comes from — originally, it might have indicated the price of 100 nails.


This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.

Frequently Answered Questions

What's an Automatic Colt Pistol and how does it relate to bullet sizes?
An Automatic Colt Pistol (ACP) refers to the type of ammunition used in Colt semi-automatic pistols, which are of specific bullet sizes. For example, a .45 ACP means the bullet diameter is .45 inches, tailored for a Colt pistol's chambering.
How do rifle calibers affect a rifle's Performance?
Rifle calibers determine the diameter of the bullets a rifle can fire, which directly influences the bullet's speed, trajectory, and stopping power. Different calibers allow rifles to be versatile in various shooting activities, from small-game hunting to long-range target shooting.
Why is bullet weight an important factor to consider?
Bullet weight, measured in grains, impacts the bullet's velocity, energy, and recoil. A heavier bullet will generally travel slower but can deliver more impact, whereas a lighter bullet is faster and may have less recoil, affecting shooting precision and performance.
What's the difference between open tip bullets and hollow point bullets?
Open tip match bullets are designed with a small opening at the tip for aerodynamic stability, often used for sniping, whereas hollow point boat tail bullets feature a larger cavity designed to expand upon impact for maximum energy transfer, which makes it a popular hunting round and self-defense choice.
What is a pistol round?
A pistol round refers to the complete cartridge used in handguns, which typically includes a bullet, casing, powder, and primer. It's generally shorter and designed for use in the less spacious chambers of pistols, as opposed to longer rifle cartridges.
Can you explain the types of shotgun ammunition available?
Shotgun ammunition comes in various forms, including birdshot with small pellets for hunting birds, buckshot with larger pellets for bigger game or self-defense, slugs for single-projectile shooting, and less-lethal options like bean bags for crowd control.
What are key considerations for concealed carry ammunition?
When choosing concealed carry ammunition, factors like bullet weight, caliber size, recoil, reliability, and the type of bullet tips — such as hollow point or full metal jacket — are important to ensure effective self-defense while maintaining control and comfort.
How does a pellet gun round differ from traditional ammunition?
A pellet gun round is typically a small, diabolo-shaped projectile made of lead or alloy, fired by compressed air or CO2, rather than the explosive propellant used in traditional firearm ammunition. Pellet gun rounds are used for recreational shooting, small game hunting, and pest control.
What does the term "go-to round" mean?
The term "go-to round" refers to a cartridge that a shooter prefers and relies upon due to its proven reliability, effectiveness, or suitability for a particular purpose, such as self-defense, hunting, or competition shooting.
Why is a firing pin essential for a firearm to function?
When the trigger is pulled, it strikes the primer of the cartridge case, igniting the gunpowder and ultimately firing the bullet. Without a functioning firing pin, a firearm cannot discharge a round.