The exact 20 gauge slug diameter typically measures 0.615 inches, which converts to 15.62 millimeters. Like 12 gauge shotgun shells, 20 gauge slugs vary slightly depending on whether they are designed for smoothbore barrels (such as traditional Foster lead slugs) or rifled barrels (such as modern sabot slugs). This measurement refers specifically to the width of the projectile itself, which differs from the internal dimensions of the barrel and the traditional “gauge” sizing system. Specifically, a standard 20 gauge shotgun barrel utilizes a bore diameter of approximately 0.615 inches, though choke constriction can alter how the slug exits the muzzle.
20 Gauge Slug Diameter
| Specification | 20 Gauge Slug |
| Nominal Slug Diameter | 0.615 inches |
| Slug Diameter In Inches | 0.575″ to 0.615″ |
| Slug Diameter In Millimeters | 14.60 to 15.62 mm |
| Groove / Bore Diameter | ~0.615 inches |
| Typical Slug Weights | 275 gr to 438 gr (approx. 5/8 oz to 3/4 oz) |
| Case Length (Unfired) | 2.75″ or 3.0″ |
| Maximum Pressure | 12,000 to 14,000 PSI |
What Is The 20 Gauge Slug Diameter?
The 20 gauge slug diameter is the physical width of the heavy projectile designed to fire out of a 20-gauge shotgun. Historically, “gauge” refers to the weight of a round lead ball that equals one-pound divided by the gauge number (meaning a 20 gauge ball weighs one-twentieth of a pound of lead, which mathematically yields a diameter of approximately 0.615 inches).
20 Gauge Slug Diameter In Inches
According to industry standards, traditional smoothbore lead Foster slugs for a 20 gauge typically measure around 0.575 to 0.615 inches to allow safe passage through various shotgun chokes. Meanwhile, sabot slugs—which are encased in a plastic sleeve designed to fire out of rifled shotgun barrels—utilize a smaller internal projectile (frequently a .451-inch or .452-inch bullet) nestled inside a 0.615-inch sabot cup that peels away after leaving the barrel.
20 Gauge Slug Diameter In Millimeters
For those using metric measurements, the standard 20 gauge bore and traditional slug diameter in mm is approximately 15.62 mm. Because 1 inch equals 25.4 millimeters, multiplying 0.615 inches by 25.4 yields 15.621 mm. Sabot slugs utilizing sub-caliber rifle projectiles will measure according to their internal bullet width (such as 11.48 mm for a .452-caliber projectile).
20 Gauge Slug vs Bore Diameter
Unlike rifled firearms where the bullet is deliberately wider than the bore to bite into rifling lands, traditional smoothbore 20 gauge slugs match or slightly under-run the barrel diameter (0.615 inches). Foster-style slugs feature hollow bases that expand under gas pressure to seal the bore, and soft lead fins that compress safely if fired through a choked shotgun barrel, preventing dangerous over-pressurization.
20 Gauge Slug Weights
Shotgun slugs are significantly heavier than standard rifle bullets, typically measured in fractions of an ounce or grains. Standard 20 gauge slug weights range from 5/8 ounce (approx. 273 grains) up to 3/4 ounce (approx. 328 grains) and 7/8 ounce (approx. 383 grains). Changing the weight alters the recoil and trajectory, but the outer shell and sabot dimensions are built specifically for a 20 gauge chamber.
Slug Diameter vs Other Large Calibers
Shooters frequently compare shotgun slugs to large-bore rifle cartridges. Here is how traditional 20 gauge dimensions compare:
| Projectile / Cartridge | Diameter In Inches | Diameter In mm |
| 20 Gauge Slug (Traditional) | 0.615″ | 15.62 mm |
| 12 Gauge Slug | 0.729″ | 18.52 mm |
| .50 BMG | 0.510″ | 12.95 mm |
| .450 Bushmaster | 0.452″ | 11.48 mm |
| .308 Winchester | 0.308″ | 7.82 mm |
Notice that while a 20 gauge slug is narrower than its 12 gauge counterpart, it remains vastly wider than even heavy-duty anti-material rifle rounds like the .50 BMG.
Slug Weight and Types
Ammunition manufacturers produce distinct types of 20 gauge slugs for different barrel configurations:
| Slug Type | Example Load | Typical Weight | Advertised Velocity |
| Foster (Rifled) Slug | Remington Slugger 20ga | 5/8 oz (273 gr) | 1,600 fps |
| Sabot Slug (Rifled Barrel) | Hornady SST 20ga | 250 gr (.452″ bullet) | 1,800 fps |
| Buckhammer / Solid Lead | Federal Power-Shok 20ga | 3/4 oz (328 gr) | 1,500 fps |
Velocities represent typical factory test barrel data.
Dimensions
To understand how 20 gauge slug shells fit into standard chambers, here are the general dimensional specifications:
| Dimension | Measurement |
| Nominal Bore/Slug Diameter | 0.615″ (15.62 mm) |
| Standard Chamber Lengths | 2.75″ and 3.0″ |
| Hull Base Diameter | ~0.690″ |
| Maximum Operating Pressure | ~12,000 to 14,000 PSI |
20 Gauge Slugs: Smoothbore vs. Rifled Barrels
Matching the slug type to your shotgun’s barrel is essential:
- Foster / Traditional Slugs (Smoothbore): Designed for smoothbore barrels. Their exterior fins provide aerodynamic stability rather than interacting with barrel rifling.
- Sabot Slugs (Rifled Barrels): Require a dedicated rifled 20 gauge barrel. The barrel’s internal rifling spins the plastic sabot cup, which stabilizes the smaller internal bullet. Firing sabots through a smoothbore destroys accuracy, while firing lead Foster slugs through a tightly choked rifled barrel causes excessive fouling.
20 Gauge Slug Diameter: Exact Measurement
The exact traditional 20 gauge slug diameter is 0.615 inches, or 15.62 millimeters.
While smoothbore slugs are sized to match the 0.615-inch interior of the barrel, modern sabot variants house a smaller sub-caliber bullet inside a 0.615-inch discarding plastic jacket. Operating at moderate pressures (around 12,000 to 14,000 PSI), 20 gauge slugs deliver manageable recoil combined with exceptional terminal effectiveness for medium-to-large game hunting.
FAQs
What is the diameter of a 20 gauge slug?
The traditional nominal diameter is 0.615 inches.
What is 20 gauge slug diameter in mm?
In metric measurements, a standard 20 gauge bore and slug diameter translates to 15.62 mm.
Are 20 gauge slugs smaller than 12 gauge slugs?
Yes. A 20 gauge slug (0.615″) is noticeably smaller in diameter than a 12 gauge slug (0.729″).
Can you shoot 20 gauge slugs through a choked barrel?
Traditional lead Foster slugs can generally be fired through open chokes (like Improved Cylinder or Modified), but you should avoid shooting them through tight chokes (like Full) unless specified by the manufacturer. Sabot slugs perform best out of cylinder or rifled choke tubes.
What is the weight of a 20 gauge slug?
Most standard 20 gauge slugs weigh between 5/8 ounce (273 grains) and 3/4 ounce (328 grains).
How does a 20 gauge slug compare to a .50 BMG in size?
The 20 gauge slug is physically much wider (0.615 inches) than a .50 BMG projectile (0.510 inches), though the .50 BMG operates at vastly higher rifle pressures.