What Caliber Is a 308 Rifle?

The Short Version

• A 308 rifle has an inside barrel width of exactly 0.308 inches, which equals roughly 7.82 millimeters.
• The number 308 is the caliber itself, which places the rifle squarely within the popular 30-caliber family of firearms and airguns.
• The label 308 describes the diameter of the projectile (the bullet or lead slug fired from the gun), not the length of the ammunition or the amount of gunpowder inside it.
• You will find this bore size in traditional powder rifles like the .308 Winchester, as well as in high-power air rifles built to fire heavy lead slugs.

The Exact Physical Size of a 308 Rifle

When you see the number 308 stamped on the side of a rifle barrel or printed on a box of ammunition, it refers to a specific physical measurement. In the United States, caliber usually measures the width of a barrel in decimal fractions of an inch. A 308 rifle barrel measures 0.308 inches across from side to side on the inside.

To picture what 0.308 inches looks like in daily life, think of a standard wooden pencil. The lead and wood of an unsharpened pencil measure just barely wider than 0.300 inches across. If you take three standard quarters from your pocket and stack them flat on top of each other, the thickness of that small coin stack is very close to 0.308 inches. It is also slightly under five-sixteenths of an inch on a standard household tape measure.

To understand why that measurement matters, you need to look at the bore (the inside tube of the barrel). Gun barrels are not completely smooth pipes. Manufacturers cut spiral grooves into the metal wall inside the tube. The raised metal ridges between these spiral grooves are called lands. As the projectile travels down the bore, these raised ridges bite into the soft outer metal of the projectile. This spiral grip forces the projectile to spin rapidly, just like a well-thrown football, so it flies straight and stable through the air.

When a manufacturer labels a rifle as a 308, that number represents the groove diameter. The barrel starts with a hole drilled to 0.300 inches across. Then, the spiral grooves are cut four-thousandths of an inch deep on each side. That brings the total distance between opposing grooves to exactly 0.308 inches. Because the bullet or lead slug matches this 0.308-inch groove dimension, it fills the barrel completely, trapping the high-pressure gas behind it to push the projectile out at full speed.

Metric Comparison: What 308 Measures in Millimeters

In most parts of the world outside the United States, engineers measure barrel sizes using the metric system instead of inches. If you convert 0.308 inches directly into metric units by multiplying by 25.4, you get 7.82 millimeters. Yet, if you look at military ammunition crates or international shooting charts, you will constantly see 308 rifles linked directly to the number 7.62 millimeters. This difference causes endless confusion for new shooters.

The difference comes down to where the barrel was measured. American commercial gun makers usually name the caliber by measuring the widest point of the barrel, which is the distance across the deep grooves (7.82 millimeters, or 0.308 inches). Military naming conventions, especially within NATO and European arsenals, often measure the narrowest point of the barrel before the grooves are cut. That narrow point across the tops of the raised lands measures 7.62 millimeters, which equals 0.300 inches.

Both names describe the exact same barrel dimensions, just measured from different points on the spiral metal pattern. This is why the commercial cartridge known as the .308 Winchester uses the exact same bullet diameter as the military cartridge known as 7.62x51mm NATO. Both cartridges fire projectiles that are 0.308 inches (7.82 millimeters) wide. When you hear someone talking about a 7.62 rifle, they are talking about the metric equivalent of the American 30-caliber bore.

The 30-Caliber Family: Sorting Out Similar Sizes

A 308 rifle belongs to a larger group known as the 30-caliber family. People often use "30-caliber" as a general umbrella label for any rifle that fires a bullet roughly three-tenths of an inch wide. However, you cannot assume that all 30-caliber rifles take the same ammunition. While many of them share the exact same 0.308-inch projectile width, their brass cartridge cases are completely different sizes and shapes.

Consider a few famous members of this rifle family to see how they differ:

• .30-30 Winchester: This classic lever-action rifle cartridge fires a 0.308-inch bullet, but the brass case is slender, rimmed at the base, and holds a modest amount of powder designed for shorter shooting distances.
• .30-06 Springfield: This historic cartridge served American troops through both World Wars. It fires the exact same 0.308-inch bullet as a modern .308 rifle, but its brass case is much longer, allowing it to hold more gunpowder.
• .300 Winchester Magnum: This heavy-hitting hunting cartridge also uses a standard 0.308-inch bullet. However, the brass case is extra wide and reinforced with a thick metal belt at the base, generating much higher pressure and heavy shoulder recoil.
• .300 AAC Blackout: This compact round uses a trimmed-down case designed to fit inside standard military-style carbines. It fires a 0.308-inch bullet, but it moves much slower and uses a very short case.

Because the brass cases vary so dramatically, ammunition from one 30-caliber rifle will not safely chamber or fire in another. If you try to force a longer cartridge like a .30-06 into a standard .308 rifle, the rifle action will not close. If you accidentally fire a shorter cartridge in the wrong chamber, the brass case can burst, spraying hot gas backward and damaging the rifle. The barrel width is identical, but the ammunition shape is not.

Powder Firearms Versus Big-Bore 308 Air Rifles

While most people associate 308 rifles with traditional gunpowder cartridges, this caliber has also become a major standard in the airgun world. Modern big-bore air rifles do not shoot tiny steel BBs or lightweight .177 pellets. Instead, they feature heavy, rifled steel barrels built specifically to shoot large 0.308-inch lead projectiles.

These large airguns are usually PCP rifles (rifles filled with high-pressure air from a special hand pump, compressor, or scuba tank). Instead of relying on exploding gunpowder, they use a blast of compressed air stored at several thousand pounds per square inch to launch a slug (a solid airgun bullet shaped like a traditional firearm bullet). These lead slugs have solid, smooth sides that engage the barrel rifling in the exact same manner as a powder-fired bullet.

Standard backyard air rifles typically shoot lightweight, waist-shaped lead pellets in .177 or .22 calibers. Those smaller airguns produce modest power suitable for paper targets, tin cans, or small garden pests. In contrast, a .308 air rifle fires a projectile weighing anywhere from 100 to 150 grains. They produce substantial muzzle velocity (the exit speed of the projectile) and high foot-pounds of energy (the real punch or humane knockdown power delivered on impact).

Because a .308 air rifle produces knockdown power similar to small firearm cartridges, it requires the same strict safety rules. You cannot shoot a .308 air rifle against a simple wooden board or a thin plastic target stand. You must shoot against a rated backstop berm of thick earth or a heavy commercial steel trap that completely absorbs the heavy lead slugs without allowing dangerous ricochets.

Matching Ammo to the Barrel: A Step-by-Step Checklist

Buying the wrong ammunition or the wrong airgun slugs can ruin an afternoon at the range or create an expensive safety hazard. Before you purchase any ammunition or load your rifle, follow this simple checklist to verify an exact match.

1. Read the barrel stamp: Look closely at the steel barrel just forward of where the action opens. Every reputable rifle maker stamps the exact caliber directly into the metal. Look for clear text such as ".308 Winchester" or ".308 Air Only." Never guess based on what the gun looks like from the outside.
2. Match every word on the ammunition box: Do not buy a box simply because it has the number 30 somewhere on the label. A box labeled ".300 Savage" or ".307 Winchester" is not the same thing as ".308 Winchester." Every word and number on the box must match the metal stamp on your barrel.
3. Verify airgun slug diameter: If you shoot a big-bore air rifle, pay close attention to the decimal sizing on the slug box. Some airgun barrels are sized for true .30-caliber pellets (which measure 0.300 or 0.303 inches), while others are true .308 barrels. An undersized 0.300 slug fired down a 0.308 barrel will fit loosely, letting air leak around the sides and causing poor accuracy. An oversized slug can jam tightly in the bore.
4. Check projectile clearance if you cast your own: Hand-loaders and airgun hobbyists who pour their own lead projectiles should always measure finished slugs using a digital caliper. Confirm that the outer diameter measures exactly 0.308 inches before loading them into your magazine.
5. Store your ammunition separately: If you own multiple rifles in the 30-caliber family, never store loose cartridges mixed together in the same shooting bag. Keep them in their original labeled boxes, and store your collection securely. Keeping your gear locked away in a sturdy cabinet with a dependable electronic gun safe lock prevents unauthorized access and stops you from accidentally grabbing the wrong ammunition box in a hurry.

FAQs

Can you shoot .308 Winchester ammunition in a 30-06 rifle?

No, you cannot safely fire .308 Winchester ammunition in a .30-06 rifle. Although both cartridges use bullets that measure 0.308 inches wide, the .308 brass case is roughly half an inch shorter than the .30-06 chamber. If you pull the trigger, the shorter cartridge will sit unsupported inside the firing chamber, which can rupture the brass case and cause severe injury.

Can you shoot standard .30-caliber pellets in a .308 air rifle?

Standard .30-caliber airgun pellets do not fit properly in a true .308 air rifle barrel. Traditional .30-caliber pellets are manufactured to measure 0.300 or 0.303 inches across, which leaves them slightly too narrow for a .308 bore. Because the pellet fits too loosely to grip the barrel rifling, compressed air will escape around the edges and your shots will fly erratically.

Is a .308 rifle suitable for beginner target shooters?

A .308 rifle is very accurate and easy to learn on, but traditional powder models produce noticeable shoulder recoil that can cause new shooters to flinch. If you are learning basic shooting fundamentals from scratch, starting with a smaller bore or a light training rifle makes practice much more comfortable. Once you understand the basics of sight alignment and trigger press, stepping up to a .308 is straightforward.

What is the difference between .308 Winchester and 7.62x51mm NATO?

Both cartridges share the exact same external dimensions and fire identical 0.308-inch bullets. The practical difference is that commercial .308 Winchester ammunition is manufactured to slightly higher maximum pressure levels, while military 7.62x51mm brass cases have thicker internal walls. Most modern rifles stamped for .308 Winchester can safely fire military surplus 7.62x51mm rounds without any trouble.