Hey, here’s a link to one system likely proven out in makers faires…
https://rcwarshipcombat.com/resources/3-d-printed-torpedo-gun.29/
A pair of these could also arm escort carriers I’ll bet.
Anyway, a lot of designs and such are out there… so it’s assembling the bike, not inventing the wheels 
lol and we do have a gunsmith that belongs to the hive… just saying 
In the rule it specified caliber however not shape… lol im all for making miniature scaled rounds over bbs and rifling the bore, even though short
Well… lets get the proven stuff going before the experimental eh? I am pretty certain the missing sections of the rules specifiy round shape. Problem with the other shot is… no lead, no copper for marine life stuffs. Problem with regular bbs is that they have both a flat and a coating. So, smoothbore works… and the rounds do skip a bit across the pond. You can find references to when they did use several bullets and or bearings for torpedoes… the problem is the damage quickly gets to out of scale… and the other huge part of that is if you can use it, the other side can too.
Would be really good to consult with a bonafide gunsmith on a few issues legal like for sure.