Okay this really got me thinking though, so I looked around to see if I could find anyone who's done super short 12g testing recently.
Here's what I found: Small Arms Review tried it, minimum length, 6 7/8".
Muzzle velocity: 1,117 fps.
From 30" down to 6 7/8" they lost a little under 500 fps. Well, peak speed wasn't at 30 so it depends a bit on how you want to describe everything in plain English; recently I had an argument with some engineers because I casually commented that smaller structures are typically more rigid than larger ones and I quickly realized that the amount of interest they had in exporting mathematical precision to conceptual conversation radically exceeded my own. So take the numbers how you will.
Bottom line: from 30" down to 12 they only lost about 200fps. Over the next 5", they lost about 250fps.
Spread was smallest out of the 18" barrel : Remington 9 pellet 00 had a 8.5" spread at 15y.
The super Shorty was 17" at that same distance which is actually better than I would have guessed. Still too big IMO but I thought it would be worse.
https://smallarmsreview.com/display....idarticles=111