Barrels & Sights


The Shotgun Barrel: The barrel is the portion of the shotgun that aligns and controls the fired shot charge to the point of aim.

Chamber: The chamber is located internally at the breech end of the barrel (the end closest to the shooter when firing). It is the first 3" to 4" and is machined slightly larger in diameter than the bore (barrel's interior) to allow for a shell casing to slide in and out during loading and unloading.

Bore: Refers to the interior of a barrel, forward of the chamber. The bore diameter determines the gauge of a shotgun.

Choke: The choke is located internally and at the muzzle end of the barrel (the end farthest from the shooter when firing.) Depending on the choke system, it is approximately the last 1" to 3" of a bore. The amount of the choke's constriction determines the diameter of the shot pattern at a given distance.

The inside diameter of the barrel's bore determines the gauge of a shotgun.

10 ga.  12 ga.  16 ga.  20 ga.  28 ga.  .410

Muzzle view of bores, to scale but not  actual size


Gauge: Shotguns are offered in various gauges (ga.) or bore sizes. Mossberg® offers 12, 20 and a .410 bore. These are the best-selling shotgun bore sizes. Other gauges are available, such as the 10, 16 and 28 gauge, but are less popular.

Amaze your friends - explain this at your next social gathering: The gauge of a shotgun barrel was originally determined by the number of identically-sized lead balls that have the same diameter as the bore, and weigh one pound. In other words, 12 lead balls, each with the diameter of a 12-gauge bore, weighs one pound; 20 equally sized balls, whose total weight is one pound, would identify the 20 gauge. The .410 is the only exception; it's not even a true gauge. Its bore is the actual dimension of .410 of an inch, and, therefore, it is improper (except in hunting camps where language skills are excused) to refer to the .410 as a gauge (or you can take the other route, and ask your dealer for a box of 67-gauge shells.)

Shotshell in a Nutshell: A shotgun is designed to fire a shotshell, which is a round of ammunition containing multiple pellets (also called shot or BB's). Once fired, the distance between the leading and trailing pellets is called the shot string. The width and density of the area the pellets cover at the point of impact is called the pattern.

Shell Lengths: The standard (U.S.) shell length for 12 and 20-gauge ammunition is 2-3/4" (.410 is 2 1/2") - measured after firing. Ammunition manufacturers also offer 2 3/4" 'magnums' and 3" magnum shells (all 3" are magnum shells.) Each shotgun barrel has its chamber length and gauge stamped on the barrel. If the barrel reads, "Chambered for 2-3/4" and 3" shells" - then the chamber is long enough to handle a SAAMI standard 2-3/4" and the longer 3" (magnum) shell. Ammunition manufacturer's catalogs and websites offer detailed ammunition information. Check to be certain that the ammunition selected is the same type of cartridge as designated on the left side of the barrel.

Interchangeable barrels and their ability to handle a wide assortment of shotgun ammunition makes the shotgun the most versatile firearm ever invented. Switching a Mossberg® pump barrel from one field style to another or even to a muzzleloader barrel takes only seconds, and no tools are required.

Mossberg® pump-action barrels are interchangeable within model, capacity and gauge.

Traditional "Smooth Bore" Field Barrel: "Smooth bore" describes the majority of shotguns whereby the inside of the barrel has a smooth finish. This has been the traditional bore treatment for all shotgun barrels delivering a load of shot. The most dominant smooth-bore barrel lengths are between 24" and 30" and generally include an interchangeable choke tube system. The shorter length barrels are more popular for hunting in woodlands and brush, while longer barrels are favored for hunting in open fields or marshes. Some hunters prefer the smooth swing of a long barrel, others prefer a fast, compact swing. Go with the barrel length you are most comfortable with.

Important features and benefits of Mossberg® field barrels:

Ventilated Rib (a.k.a. Vent rib, VR, rib): An elevated surface on top of the barrel that extends the full length of the barrel. The rib provides a flat plane for sighting the gun and minimizes heat waves in the line of sight.

Mid-point and front bead sights are standard on Mossberg® vent rib field barrels to assist a shooter's sighting alignment. Placing the brass mid-point in the same position behind the white front sight will help maintain a consistent point of aim. Pattern test your gun to establish your ideal sight placement before heading out to the field.

Ported Barrels: Formerly reserved for customized competition guns, porting allows some of the gasses to escape upwards, near the "muzzle" end of the barrel. This offsets some of the "muzzle jump" that occurs when a shell is fired. Porting also reduces felt recoil, helping the shooter maintain control for the next shot.

The Shotgun's Newest Frontier:Centerfire rifles with their long-range accuracy, have been the accepted firearm for deer hunting in the United States.  Recently, however, many deer hunting areas have been designated "shotgun only." This has created a new challenge for the shotgun and the hunter: to establish an unprecedented benchmark for riflelike performance.

The Fully Rifled Slug Barrel: Mossberg® has been making rifles and rifling barrels for years, therefore, it  seemed obvious:  If rifled barrels improved the accuracy of rifles, then why not a rifled bore for a shotgun barrel. A rifled bore has a shallow groove machined into the full length of the bore with a very slight twist. The grooves impart rotary motion to a projectile, which results in improved and consistent accuracy (the uncut surface areas are called "lands.")

Mossberg® 12-gauge fully rifled barrels have a 1 in 36" right-hand twist.

Intent on providing the ultimate in slug shooting accuracy and after extensive testing of barrels and ammunition, Mossberg® began producing fully rifled shotgun barrels in concert with distributing newly engineered sabot style slug ammunition. Shortly thereafter, the major ammunition manufacturers began producing their own sabot style slugs specifically for the rifled barrels, while scope manufacturers designed optics to withstand a shotgun's recoil. A combination of innovative products from within the firearms industry was the dramatic response to shooters' demands for long-range shotgun accuracy. The results were impressive, taking the average bushel-basket group at 50 yards, to 3" groups at 100 yards. Complementing features like improved rifle-style sights, integral scope bases and Dual-Comb® stocks were developed specifically for the new slug guns.

While sabot-style slugs perform best in fully rifled barrels, traditional rifled slugs have also improved immensely, and are recommended for smooth-bore barrels.

There are two types of slug barrels, rifled and smooth bore. A slug is a single projectile. And to deliver it to a specific point on a target requires concentration and a more exact aiming method than the typical bead sights. Therefore, one important and distinguishing trait of a slug barrel is the sighting system. Here are a few of our sighting options:

Rifle Sights (iron sights, buckhorn)  A firearm is aimed by properly aligning a single front sight post or blade between a rear sight notch at a given target. The same style of sights were used on rifles long before shotguns used them, hence the name, "rifle sight."

View of fiber optic sights looking down the barrel at a point of aim. The "illuminated" green front sight post is more easily aligned with the rear blade notch, when guided by the two "illuminated" red fibers.

Fiber optic sights are an enhancement of the typical rifle sights. They are essentially the same as rifle sights, except they employ small fibers of colored plastic to gather ambient light. The ends of each fiber strand appear brightly illuminated and are strategically positioned to assist in the sight alignment process. The fiber optic sighting concept was first used and proven by bow hunters who found them especially helpful during the early morning and evening hours and in general low-light conditions.

Ghost Ring® Sights (GRS) are similar to a rifle sight, except a small hole in the rear sight aperture is used for alignment with the red front sight blade. The eye's natural instinct to quickly obtain the clearest view helps center and align the front and rear sight picture. Ideal for fast, short-range target acquisition, the GRS was originally designed for law enforcement use.

View of GRS looking down the barrel at a point of aim. The ring nearly disappears (ghosts) when the eye focuses on a target farther in the distance.

Scopes (telescopic sights) valued for their target magnification ability and sighting consistency, scopes proved themselves as the preferred sighting device for multi-range accuracy. Most Mossberg® shotgun receivers are "Drilled and Tapped" (D&T) at the factory to accept scope bases for scope mounting.

Due to the heavy recoil of a shotgun, be sure the scope is rated for shotgun use, and be sure to maintain an adequate distance between the rear of the scope and your face at all times.

Mossberg® Trophy Slugster® barrels have an integral scope base welded directly to the fully rifled barrel. Once the scope is mounted to the barrel and they are zeroed-in, the scoped barrel can be removed from the shotgun to clean or interchange barrels and be returned time after time, without having to remove the scope or re-zero the barrel.

Please remember: No one can call a shot back - slugs have a range of over a half-mile.

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