The various shotgun parts each provide for certain functions of a shotgun. For instance, the trigger is just one of the shotgun parts that propels the ammunition through the barrel of the gun. The length of the barrel directly effects the firing power of the weapon and the speed at which the ammunition is propelled out of the weapon.
Sawed off shotguns can be especially deadly because the ammunition is propelled faster than it would be if the barrel was complete. In addition, sawed off shotguns are easy to conceal, which makes them extremely dangerous for law enforcement in certain situations, such pulling over a vehicle that may contain a sawed off shotgun.
In the United States, it is illegal to posses a sawed off shotgun that has a barrel length of less than eighteen inches, unless the individual has obtained a taxed permit from the ATF. That permit requires a background check and payment of an excise tax for that weapon. However, each state has different laws which govern the types of weapons that residents are allowed to own and a state may forbid ownership of a sawed off shotgun, simply because of the ease with which it can be concealed.
Sawed off shotguns can be easily hidden and they are very powerful. However, they may not be as accurate as shotguns that have the longer barrel. That inaccuracy is another danger present by use of this type of weapon.