This is one of those things, that seems like it has just always been in my knowledge. A recent comment about rifles on a question about shotguns got me thinking, it really is not all that simple.
They make slugs that you can fire in a shotgun, and they make shot you can fire in a rifle.
If they can both shoot either slugs or shot, is there a difference? Are the names rifle and shotgun interchangeable? If there is a difference what are key points that distinguish them?
The term 'rifle' comes from the fact that the barrel of a rifle has spiral grooves (called rifling) cut into the inside. When the bullet is fired it is forced into these grooves which in turn cause it to spin as it travels down the barrel. This spin around its long axis creates a gyroscopic effect which stabilises it in flight dramatically improving its accuracy and the consistency of its trajectory.
This axial stabilization also means that the bullet can be made with a more aerodynamically efficient shape (typically a cone or cylinder) as it is much less prone to tumbling end over end in flight.
A consequence of this is that a rifle bullet is a tight fit in the barrel and so very little of the gas pressure generated by the propellant can get past it compared to a shotgun and so typically a rifle requires a rather heavier barrel than a shotgun as it must contain greater gas pressures even though a shotgun shell may have a greater mass of propellant.
A shotgun is designed to produce a spread of shot over short to moderate ranges which makes it easier to hit small fast moving targets, such as small game and especially birds in flight. the limitation is that its stopping power and accuracy decrease quickly with range.
In a military context shotguns have the advantage that they have high stopping power at very short ranges, can hit a target without careful aim* and have a very versatile selection of rounds for different purposes. They are also particular useful for breeching unarmored doors either with buckshot, solid slugs or specialist breeching rounds there are also specialist rounds for firing gas and pyrotechnic munitions through light doors and walls as well as for shattering windows (relatively) safely.
*In response to comments I will clarify this point by adding that shotguns are well suited to quickly acquiring and hitting fast moving targets of opportunity both because of their spread pattern and general handling characteristics. Here 'careful aim' was perhaps the wrong choice of words but the point is that they work well for reflexive type shooting styles both in the context of game shooting (eg birds flying from cover) and certain military contexts. This is especially the case over moderate ranges in close terrain (eg dense forest or outdoor urban terrain) shotguns may be a very effective weapon as an initial counter to ambushes or unexpected contact with the enemy at short range.
Solid slugs in shotguns are normally primarily intended as anti-material rounds for use against light vehicles etc at fairly close range but they can also be used to improve effective range and accuracy against personnel.
Solid slugs are also commonly the only allowed munition for hunting large game on public land. Rifle rounds travel very far before hitting the ground due to their high muzzle velocity relative to shotgun slugs, so they are more dangerous in the case of a miss. Buckshot is typically banned on public land as it doesn't group reliably and stray pellets can go on long and magical journeys into the woods.
Pump action shotguns also have the advantage that a single round can be loaded and fired very quickly from an unloaded weapon. For all of these reasons shotguns are often used as primary or additional weapons by at least one member of units specializing in close quarters, urban or hostage rescue.
In contrast rifles are accurate and effective at much greater ranges. Although shot rounds for rifles do exist they are pretty rare.
In the U.S. there are legal definitions provided by code (18 USC 921) and regulation (27 CFR 478.11), which are somewhat derived from the customary definitions.
A shotgun is designed to shoot shot (i.e., multiple ball projectiles) through a smooth bore. Yes, you can buy rifled shotguns designed to fire a single projectile, but they still shoot shotgun shells, which are large caliber and produce low maximum chamber pressures. I believe the SAAMI MAP (Maximum Average Pressure) for shotshells is always under 12kpsi. And the smallest standard shotshell I'm aware of is .410.
A rifle is designed to shoot a single projectile at high pressures through a rifled barrel. It is common to see MAP for rifle cartridges over 60kpsi. And it is extremely uncommon to see a man-portable rifle with a bore larger than .50 caliber*. (And perhaps even less common to find shooters willing to get behind one's trigger ;)
So a decent definition could distinguish these firearms primarily by design pressure.
It is true that you can find rifle ammunition loaded to lower pressures. You can even find some corner cases where a rifle is loaded to fire more than one projectile – e.g., the M198 "duplex." You managed to find a .22LR "shot shell." That's just a weird product: It's a low pressure round, and its effective range is measured in feet because rifling throws shot into a ridiculously wide "pattern." I.e., you can fire shot through a rifle, but other than as a point-blank "snake" load, or just out of curiosity, you wouldn't want to. It really fouls the rifling, and it doesn't let you hit anything at a distance with any reasonable probability.
*ATF exemptions exist, so that's not just because in general rifles over .50 are classified as Destructive Devices.
The distinction between rifle and shotgun can get convoluted if only considering shot vs single projectile. Shotguns can be loaded with rifled slugs, rifles can fire shot shells. CCI has marketed shot shells for the 22LR, 22Mag, 9mm Luger, 40S&W, 44 Mag, and probably others (those are just the ones I've bought). Yes, I've seen and/or owned rifles in those calibers, but depending on the definition, a pistol is also a rifle. I also have a smoothbore 22 Short "rifle", similar, I am to understand, to the ones my father used to shoot skeet as a Boy Scout many years ago; it is marked 22 Shotshell Only. In modernity, the Taurus Judge pistol can be loaded with both/either 44 Long Colt or .410 shot/slug shells. Other such examples exist. I would make the distinction based on pressure and velocity rather than projectile type. Shotguns are designed only to handle low pressure and low (generally < 1600fps) velocities, while rifles (and pistols) are designed to handle high pressures to generate high velocities (up to 4400+ fps). Many pistol rounds fall into the velocity range of shotguns, but do so using much less powder by generating high pressure. Rifling is a distinguish characteristic of rifles, smooth bores of shotguns, but there are exceptions. Seems such exceptions would have to be taken on a case by case basis. One of my favorite guns is both; over/under with 22 mag on top, 20 gauge on bottom. Great for turkey hunting, where allowed. BTW, agreed, shot shells, especially those with free floating shot (CCI encapsulates the shot in a plastic casing) can deposit lead in a barrel quite quickly.
I believe that the term presence of Rifleing in the bore defines the weapon as a rifle, even big caliber guns on Naval vessels are termed rifles.
Barrel length defines a pistol; it is really a short barreled rifle with less than 16" length barrel (by FFA 1934) in the USA and/or designed to be fired easily with only one hand and easily concealed.
The construction of the cartridge case is not really a defining factor as shotgun shells have been made in brass and aluminum and possibly other metals as well as paper or plastic and rifle cartridges have been made of plastic as well.
My comments are based on the physical characteristics of the barrel from a "scientific/engineering" view but not necessarily the State Criminal/Game codes. KISS
