I know that there are different chokes for different usages and rifled barrels etc.
Can I get an explanation on this?
Pattern and choke
Shot, small and round and delivered without spin, is ballistically inefficient. As the shot leaves the barrel it begins to disperse in the air. The resulting cloud of pellets is known as the shot pattern. The ideal pattern would be a circle with an even distribution of shot throughout, with a density sufficient to ensure enough pellets will intersect the target to achieve the desired result, such as a kill when hunting or a break when shooting clay targets. In reality the pattern is closer to a Gaussian, or normal distribution, with a higher density in the center that tapers off at the edges. Patterns are usually measured by firing at a 30 inch (76cm) diameter circle on a large sheet of paper placed at varying distances. The hits inside the circle are counted, and compared to the total number of pellets, and the density of the pattern inside the circle is examined. An "ideal" pattern would put nearly 100% of the pellets in the circle and would have no voids—any region where a target silhouette will fit and not cover 3 or more holes is considered a potential problem.
A constriction in the end of the barrel known as the choke is used to tailor the pattern for different purposes. Chokes may either be formed as part of the barrel at the time of manufacture, by squeezing the end of the bore down over a mandrel, or by threading the barrel and screwing in an interchangeable choke tube. The choke typically consists of a conical section that smoothly tapers from the bore diameter down to the choke diameter, followed by a cylindrical section of the choke diameter. Briley Manufacturing, a top maker of interchangeable shotgun chokes, uses a conical portion about 3 times the bore diameter in length, so the shot is gradually squeezed down with minimal deformation. The cylindrical section is shorter, usually 0.6 to 0.75 inches (15 to 19 mm). There is no good mathematical model that describes how chokes work, making the design and manufacture for chokes more art than science. The use of interchangeable chokes has made it easy to tune the performance of a given combination of shotgun and shotshell to achieve the desired performance.
The choke should be tailored to the range and size of the targets. A skeet shooter, shooting at close targets might use 0.005 inches (127 micrometres) of constriction to produce a 76 cm (30 inch) diameter pattern at a distance of 19 m (21 yards). A trap shooter, shooting at distant targets might use 762 micrometres (0.030 inches) of constriction to produce a 76 cm (30 inch) diameter pattern at 37 m (40 yards). Special chokes for turkey hunting, which requires long range shots at the small head and neck of the bird, can go as high as 1500 micrometres (0.060 inches). The use of too much choke and a small pattern increases the difficulty of hitting the target, the use of too little choke produces large patterns with insufficient pellet density to reliably break targets or kill game. "Cylinder barrels" have no constriction. See also: Slug barrel
Other specialized choke tubes exist as well. Some turkey hunting tubes have constrictions greater than "Super Full", or additional features like porting to reduce recoil, or "straight rifling" that is designed to stop any spin that the shot column might acquire when traveling down the barrel. These tubes are often extended tubes, meaning they project beyond the end of the bore, giving more room for things like a longer conical section. Shot spreaders or diffusion chokes work opposite of normal chokes--they are designed to spread the shot more than a cylinder bore, generating wider patterns for very short range use. A number of recent spreader chokes, such as the Briley "Diffusion" line, actually use rifling in the choke to spin the shot slightly, creating a wider spread. The Briley Diffusion uses a 1 in 36 cm twist, as does the FABARM Lion Paradox shotgun.
Oval chokes are designed to provide a shot pattern wider than it is tall, are sometimes found on combat shotguns, primarily those of the Vietnam War era. Military versions of the Ithaca 37 with duckbill choke were used in limited numbers during the Vietnam War by US Navy Seals. It arguably increased effectiveness in close range engagements against multiple targets. Two major disadvantages plagued the system. One was erratic patterning. The second was that the shot would spread too quickly providing a very limited effective zone.
Offset chokes, where the pattern is intentionally slightly off of center, are used to change the point of impact. For instance, an offset choke can be used to make a double barrelled shotgun with poorly aligned barrels hit the same spot with both barrels.
When slugs are fired in a standard, choked barrel, the slug is deformed by the choke as it exits. The degree of deformation is most acute with fuller chokes, which were among the most widely used in stock shotguns up until about 1990. Early shotgun slugs were "rifled" with deformable fins cast into the outside of the soft lead slug, which allowed the slug to swage down to fit the choke. With an open choke, the reduction in diameter is minimal, so accuracy does not suffer much; tighter chokes, however, deform the slug enough to impact accuracy significantly, and the impact of the slug on the choke (at velocities around 450 meters per second (1500 feet per second)) could also stretch the barrel with repeated firings.
 The first slug barrels
The first slug barrels were cylinder bore barrels (no choke) outfitted with rifle sights, which are far better suited to accurate shooting of still targets than the standard bead sight used for shooting small, moving targets with shotshells. Most pump-action and semi-automatic shotguns have barrels that can easily be changed in under a minute without tools, so having more than one barrel for a single shotgun is common. With the addition of a slug barrel, the standard shotgun used for bird hunting, skeet or trap shooting can then be used for hunting large game, such as deer at ranges of over 100 meters (100 yards).
 Rifled choke tubes
A later innovation was the rifled choke tube. It could be used in any barrel designed to use interchangeable choke tubes, and it provided rifling for the last few inches of the shotgun barrel. While this wasn't enough to impart a large amount of spin, it did impart some, and that was all that was needed for the short, fat, and inherently stable shotgun slugs of the time.
 The rifled slug barrel
The next step was the "Paradox" barrel by Hastings, a manufacturer of aftermarket rifled slug barrels. The term "paradox" had been used in the late 19th century to describe large bore guns with the last several inches of the barrel rifled, similar to a rifled choke tube. The name was chosen because shotguns are defined by their smoothbore barrels, and a "rifled shotgun" was something of a contradiction in terms--not to mention a tricky legal issue, as any firearm with a rifled barrel over 12.7 millimeters (.50 inches) is legally considered a destructive device in the United States. A BATFE ruling was obtained stating that a firearm designed to fire shotshells that was converted to fire shotgun slugs with the addition of a rifled barrel was still a shotgun, and thus not a destructive device. Now many manufacturers offer shotguns for sale with rifled barrels already installed. Bolt action and single shot break-open designs are particularly accurate, and with modern saboted slugs designed for use only with rifled barrels, the modern slug gun offers nearly the accuracy of a typical rifle.
 New slug technology
The widespread availability of rifled shotgun barrels was quickly followed by the introduction of special slugs designed for use with the rifled barrels. The short, fat, unaerodynamic Foster slug was no longer needed for its inherent stability; new slugs were smaller in diameter, usually 12.7 millimeter (.50 caliber) (compared to the 18.5 millimeter (.73 inch) bore diameter of a 12 gauge), and carried in a plastic sabot. The saboted slug had half the frontal area of the old slugs, which translated to half the drag, and double the penetration. Lighter, faster slugs were also possible, allowing for a flatter trajectory and longer range. With the wide selection of barrels, shotshells and slugs, the modern shotgun is a tremendously versatile tool.
 Shotgun slugs for safety
While shotgun slugs were originally developed as a convenience to the hunter who already owned a shotgun and did not want to purchase a rifle for hunting game, heavily populated areas now allow large game hunting only with shotguns. The limited range of the slow, fat slug--even a saboted slug--compared to a rifle bullet offers a safety advantage by limiting the maximum range. While buckshot is capable of taking deer-sized game, it is only effective at short ranges, generally under 50 meters (50 yards). A properly selected barrel and slug load can triple that range.