The right slugs can turn a 12 gauge shotgun into a .72-caliber rifle.
Shotgun shells can hold a variety of projectile types. If you can fit it in a 12 gauge hull, someone has tried to fire it out of a smoothbore. A 12 gauge shotgun can reliably fire birdshot, buckshot, and large single projectiles known as slugs.
It's an odd quirk of semantics, but while the projectile fired by a rifle is a bullet, a single solid projectile fired from a shotgun is always a slug, whether it's coming out of a rifled shotgun barrel or not.
Slugs are used by hunters and also have military, law enforcement, and self-defense applications.
Types of Slugs: Full Bore vs. Sabot Slugs
There are generally two main categories of shotgun slugs. A full-bore slug, as its name implies, fulls up most of the bore diameter as it travels down a shotgun barrel, like a bullet would.
A sabot slug uses a projectile that's actually smaller than the bore, encased in a plastic sabot. The sabot is what engages the rifling as both travel down the barrel. The plastic part drops away after leaving the muzzle, the same way a shot cup does, and allows the slug to fly on.
Let's break this extremely effective type of shotgun ammo down a little more.
These are the oldest shotgun slugs introduced for smoothbore shotguns by Karl M. Foster back in the 1930s. They were an affordable way to turn a bird gun into a deer hunting tool during the lean times of the Great Depression. They were also fairly easy to use for reloading.
Foster-type slugs are cast out of lead and include rifling grooves on the sides and a hollow rear. The weight of the lead slug near its nose creates a more stable flight pattern without the benefits of rifling spin.
The grooves on the sides don't make the projectile spin, but rather allow it to be swaged down as it passes through a shotgun's choke.
Foster slugs can be fired from most chokes, rifled choke tubes, and rifled shotgun barrels, though there is no real benefit in range or accuracy from using a Foster slug with the latter.
In the 1890s, Wilhelm Brenneke of Germany introduced the Brenneke slug. The biggest difference compared to the American Foster slug is that a Brenneke has a wad attached to the projectile that remains with it during flight to aid in stabilization.
A Bit More About Sabot Slugs
Since a plastic sabot is capable of engaging rifling in a barrel, when fired from a rifled shotgun barrel, these saboted slugs have the same advantages of spin that a rifle bullet does. This includes greater accuracy, longer range, and greater muzzle velocity.
The combination of sabot slugs and rifled shotgun barrels greatly increased the platforms capability. Today, a 12 gauge slug in a sabot is basically a .72 caliber rifle bullet (.61 caliber if you run a 20 gauge) when fired from a rifled slug barrel.
That's a lot of mass. They don't have tremendous range because of their weight, but inside 200 yards they are extremely effective and accurate. Many have hollow point designs perched in magnum shells, which can cause astonishing wound channels in big game.
What About Rifled Slugs?
Rifled slugs is another name applied to Foster slugs, because of those aforementioned side grooves, which we now know have nothing to do with rifling.
While you can fire a rifled slug from a rifled barrel, you'll just get lead in the rifling grooves and the projectile will not shoot any better than it would from a smoothbore shotgun.
You will see arguments online about this because rifled slugs do spin a little bit, more than they would if there were no grooves at all, but it's not nearly enough to create any kind of increase in stability or accuracy.
The cool thing about slugs and shotgun ammunition in general is that the same Remington 870 can be used to shoot pheasant and whitetail deer with equal efficiency. There's nothing wrong with a little more bang for your buck, eh?