Q: Passage: Under the National Firearms Act (NFA), it is illegal for a private citizen to possess a sawed-off modern smokeless powder shotgun (a shotgun with a barrel length shorter than 18 inches ( 46 cm) or an overall length shorter than 26 inches (66 cm) ) (under U.S.C. Title II), without a tax-paid registration from the Bureau of Alcohol, Tobacco, Firearms and Explosives, requiring a background check and either a $200 or $5 tax for every transfer, depending upon the specific manufacturing circumstances of the particular sawed-off modern shotgun being transferred. Gun trusts have become an increasingly popular means of registering SBS' with the BATFE. Short-barreled muzzleloading blackpowder shotguns, in contrast, are not illegal by federal law and require no tax-stamped permit, although they may be illegal under state law. As with all NFA regulated firearms, a new tax stamp must be purchased before every transfer. Inter-state transfers must be facilitated through a Federal Firearms Licensee (FFL) while intrastate transfers may be between two persons.

Question: Based on this passage, what is the shortest legal length of a shotgun barrel? Let's give stream of consciousness first:
This is the relevant information: Under the National Firearms Act (NFA), it is illegal for a private citizen to possess a sawed-off modern smokeless powder shotgun (a shotgun with a barrel length shorter than 18 inches ( 46 cm) or an overall length shorter than 26 inches (66 cm) ) ( under U.S.C. Therefore, the final answer is 18 inches ( 46 cm ).