"Shortest legal barrel is 18""
Not true in the United States.
There is no legal minimum barrel length per federal law. Under 18" requires a tax stamp. That doesn't in any way make it illegal. And no, this isn't some kind of technicality: it is the law.
You know the rest. In the books you have read
How the British Regulars fired and fled,
How the farmers gave them ball for ball,
From behind each fence and farmyard wall,
Chasing the redcoats down the lane,
Then crossing the fields to emerge again
Under the trees at the turn of the road,
And only pausing to fire and load.