You’ve Found a Firearm and Need to Know What It Shoots
Whether you inherited an old rifle, discovered a handgun in a relative’s belongings, or are simply trying to catalog your own collection, a common and crucial question arises: what caliber is this gun? Firing the wrong ammunition is not just a matter of it not fitting; it’s extremely dangerous and can cause catastrophic failure, injuring you or destroying the firearm.
The good news is that nearly every firearm manufactured in the last 150 years has its caliber stamped somewhere on the metal. The challenge is knowing where to look and how to interpret what you find. This guide will walk you through the definitive, step-by-step process to safely identify any gun’s caliber through visual inspection, measurement, and cross-referencing.
The First and Most Important Rule: Safety
Before you touch, inspect, or attempt to identify any firearm, you must ensure it is completely unloaded. Treat every gun as if it is loaded until you have personally verified it is not.
– Point the muzzle in a safe direction, away from people and anything you do not intend to destroy.
– Engage the safety if the firearm has one.
– For a semi-automatic pistol, remove the magazine first. Then lock the slide back and visually and physically check the chamber to confirm no round is present.
– For a revolver, swing the cylinder out and visually inspect each chamber.
– For a rifle or shotgun, open the action (bolt, lever, or pump) and look directly into the chamber and magazine tube.
Only once the firearm is confirmed clear should you proceed with identification. Keep ammunition in a separate room during this process.
Primary Method: Locating the Caliber Stamp
Manufacturers are required to mark the caliber or gauge on the firearm, typically in a standardized location. This is your most reliable source of information.
Where to Look on Handguns
On semi-automatic pistols, the caliber is almost always stamped on the side of the barrel or the slide. With the gun cleared and the slide locked back, you can often read it on the exposed barrel. If not, look along the frame or slide near the ejection port. Common stamps include “9mm,” “.45 AUTO,” or “.40 S&W.”
For revolvers, check the barrel itself. The caliber is frequently stamped on the side of the barrel, facing upward when the gun is held normally. You might see “.38 SPL,” “.357 MAG,” or “.22 LR.” Sometimes it’s also marked on the frame under the cylinder crane when swung open.
Where to Look on Rifles and Shotguns
On bolt-action, lever-action, and semi-automatic rifles, the caliber is typically engraved on the barrel, just ahead of the receiver. You may need good light to see it. Look for markings like “.308 WIN,” “.30-06 SPRG,” or “.223 REM.”
For shotguns, you are looking for the gauge (like “12 GA”) or the bore diameter in inches (like “.410”). This is also stamped on the barrel, often near the receiver. The chamber length (e.g., “2 3/4” or “3””) will be stamped nearby, which is critical for knowing what shell length it can safely fire.
Deciphering the Stamp
The stamp tells you the nominal bullet diameter, but the full designation is what matters for ammunition purchase. “.38 Special” and “.357 Magnum” both fire a .357-inch diameter bullet, but the cartridges are different lengths and pressures. Always buy ammunition that exactly matches the stamp.
If you see a stamp like “7.62x39mm,” the first number is the bullet diameter in millimeters (7.62mm) and the second is the cartridge case length (39mm). This is a distinct cartridge. A stamp of “.30-30 Win” indicates a .30 caliber bullet with a historical black powder charge of 30 grains.
Secondary Method: Measuring the Bore
If the stamp is worn off, illegible, or the firearm is very old or homemade, you may need to take a physical measurement. This requires precision tools and great care.
Tools You Will Need
A digital caliper is the ideal tool. A set of pin gauges (size-specific cylindrical rods) is the most accurate for determining if a bullet will fit, but is less common. For shotguns, a simple ruler or caliper to measure the inside diameter of the muzzle is sufficient to determine gauge.
Measuring Rifle and Handgun Bores
This method measures the land-to-land diameter (the raised spirals inside the barrel), which is the true caliber. It is smaller than the groove diameter (the cut-out spirals).
– Ensure the firearm is clear and safe. Remove the bolt or open the action so you can see straight through the barrel from the breech (rear) end. Measuring from the muzzle can be inaccurate due to wear.
– Using your caliper, carefully measure the distance between two opposing lands. This is a delicate operation; do not force the caliper jaws and risk damaging the barrel’s rifling.
– Convert the measurement. If you measure 0.308 inches, the caliber is likely .308 Winchester or a similar .30 caliber cartridge. A measurement of 0.224 inches suggests .223 Remington or 5.56mm.
This method gives you the bullet diameter. You will still need to determine the specific cartridge by measuring the chamber or comparing to known cartridges, which is a job for a gunsmith.
Determining Shotgun Gauge by Measurement
Shotgun gauges are an old measurement based on the number of lead balls of that bore diameter that equal one pound. You can derive it from the muzzle’s inside diameter.
– Measure the inside diameter of the muzzle in inches. A 12-gauge will measure approximately 0.729 inches.
– Use the formula: Gauge = (1 / (Bore Diameter in inches)^3) * 1.67. For a 0.729″ bore: (1 / 0.729^3) * 1.67 ≈ 12.
Common measurements: 0.410″ bore is a .410 bore (not a gauge), 0.729″ is 12-gauge, 0.775″ is 10-gauge.
Tertiary Method: Identifying by Cartridge Case or Chamber Cast
If you have a single cartridge that you know fits the firearm, you can identify the caliber from that. This is common with inherited guns where a box of old ammunition is found with them.
– Examine the headstamp. This is the marking on the base of the cartridge case. It will often have the caliber (e.g., “9mm LUGER,” “.45 ACP”) and the manufacturer.
– Use a cartridge identification chart or website. Compare the physical dimensions and shape of your cartridge to online resources.
Warning: Never force a cartridge into a chamber. If it does not drop in freely and extract easily, it is the wrong cartridge.
For a definitive identification, especially with antique or obscure firearms, a gunsmith can create a “chamber cast.” They pour a soft, low-temperature metal alloy into the chamber, creating a perfect negative model. This cast can be measured with extreme accuracy to identify the exact cartridge the gun is designed for.
Common Points of Confusion and Troubleshooting
Even with a stamp, things can get confusing. Here are the most frequent mix-ups.
The .22 Family
A stamp that simply says “.22” is ambiguous. It could be .22 Short, .22 Long, .22 Long Rifle (LR), or .22 Winchester Magnum Rimfire (WMR). These are not interchangeable. .22 LR is the most common. If it’s a rifle, it’s almost certainly .22 LR. If it’s a small revolver, it could be .22 LR or .22 WMR. Examine the chamber. A .22 WMR chamber is longer and will not fire .22 LR safely, though the reverse is physically possible but dangerous.
5.56mm vs .223 Remington
These cartridges are nearly identical but have different chamber specifications and pressure levels. A barrel stamped “5.56x45mm” or “5.56 NATO” can safely fire both .223 Rem and 5.56mm. A barrel stamped only “.223 Remington” should fire only .223 Remington ammunition, as 5.56mm may produce unsafe pressures. When in doubt, use .223 Remington.
.308 Winchester vs 7.62x51mm NATO
This is a similar situation. A .308 Winchester chamber generates higher pressure. A barrel marked “7.62x51mm NATO” can typically fire .308 Win, but the reverse (firing 7.62 NATO in a .308 chamber) is generally considered safe, though it’s best to consult the manufacturer. The stamp is your guide.
When the Stamp is in a Foreign Language or Obscure
You might find stamps like “7,62x54R” (Russian) or “9mm Parabellum” (German/Latin). Use a search engine. “7.62x54R” is a common Russian rimmed cartridge. “Parabellum” is another name for 9mm Luger. Caliber designation is a global standard, so the numbers will translate directly.
What to Do If You Still Cannot Identify It
If the stamp is gone, measurements are inconclusive, and you have no cartridge, your path is clear.
– Do not guess. Do not attempt to fire it.
– Take clear, well-lit photographs of the entire firearm, both sides, the top, the muzzle, and the breech with the action open.
– Look for any other markings: manufacturer names, proof marks (like tiny shields or crowns), serial numbers, or model numbers. These can be used to research the model, which will tell you the standard caliber it was made in.
– Take the firearm to a qualified gunsmith. This is the only safe option. A professional can use chamber gauges, casts, and reference books to provide a positive identification, often for a modest fee. This is a non-negotiable step for safety if you plan to shoot the gun.
Your Actionable Path Forward
Start with the simple visual inspection. With the gun cleared, use a bright light and a magnifying glass if needed to find the manufacturer’s caliber stamp on the barrel or receiver. This will solve 95% of cases. If the stamp is missing, carefully measure the bore diameter with a caliper from the breech end to get the bullet caliber. For shotguns, measure the muzzle diameter to calculate the gauge.
Cross-reference your findings with the firearm’s make and model online. Finally, if any doubt remains—especially if you intend to purchase ammunition and shoot it—the investment in a gunsmith’s expertise is invaluable. It ensures not only the correct identification but also a basic safety check of the firearm’s condition. Knowing your firearm’s exact caliber is the foundation of safe and responsible ownership.
