18"
Yes, it meets the minimum overall length requirements. Since it is a single shot there is no minimum length required for the barrel.
That would depend on the make and model of the shotgun in question,and the length of the barrel/barrels that came with the shotgun.
1.345m
18" for shotguns
If you mean the legal length of the barrel? it is 18 inches and for rifles it is 16 inches.
26 inches overall with an 18 inch barrel.
Unable to answer as asked. Is it already a shotgun or will it be built from a virgin receiver?
No. Barrel must be at least 18" and overall length no less than 26"
The legal minimum length of a non-rifled gun barrel in the united states is 7 inches.
According to federal law the shortest barrel length allowed for both rifle or shotgun is 16 inches.
You must consult a lawyer and the BATFE for a current and legally correct answer.