Shotguns are not a novel concept, and haven't been for hundreds of years. A few inches size difference, and even the change in loading mechanism, do not appreciably detract from the core function which remains pretty much the same. To attempt to draw a line between the two re:RKBA is to expose a bias that will not be satisfied by any examples or words, instead passionately seeking out any variance upon which to hang the "because of X, you can't exercise that right" argument.
Baloney. Take four inches off the barrel and you'll see a wider spread and a lower velocity.
"To attempt to draw a line between the two re:RKBA is to expose a bias"
Baloney. The shotgun in use at the time was 20". The law set 18" as the minimum, not because of any bias but because scumbags like Mr. Miller were concealing them and using them in criminal acts. A reasonable restriction that you don't happen to care for.
You come along and start making this 4th grade argument about "if 18" why not 17?" Fine. Make it. But not with me.