Yes, the barrels are rifled.
No. The barrels are not rifled
Although barrels were being rifled as early as the 1500s in Germany, it was not until the late 1700s that this manufacturing technique became popular.
Shotguns are smooth bored firearms, for the most part. Rifled slug barrels are available. For most of them the answer is no as they are scatter guns. meaning they are meant to hit a large area at short range. They have no real use for accuracy due to the scatter and it would be both pointless and impossible to cause the shotgun pellets to rotate.
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Rifled slugs may be safely fired thru MOST shotgun barrels, The makers of some variable chokes recommend that slugs NOT be fired thru them, and turkey guns that use the Extra Full choke aer not usually recommended for slug shooting.
Yes and no. In the early days, a rifle was defined simply as a firearm with a rifled barrel and is intended to be fired from a shoulder. Most submachine guns (SMGs) fulfill both these requirements. However, in modern times, rifles are also characterized by large bottlenecked cartridges (eg. 7.62x51mm, 5.56x45mm, 7.62x39mm), while submachine guns are characterized by the use of pistol cartridges (eg. 9x19mm, .45 ACP). So in most circles, the submachine gun is not considered a rifle.
12 g sabot slugs. Rifled slugs are for smoothbore barrels.
With very few exceptions (shotguns), most firearm barrels are rifled--even pistols.
Rifles and pistols
Yes. Rifled slugs are intended for smoothbore barrels (abot slugs aer for RIFLED barrels). Best accuracy with rifled slugs is USUALLY a modified choke, but can be safely fired thru any choke less than EXTRA full choke.
Some AUG assault rifles can be modified to be smaller and fire a smaller caliber of bullet, but these are not true submachine guns.
Rifled slugs. Sabot slugs will not be as accurate- hose are meant for rifled barrels.