Even the Krime Cannon wouldn't work. Fire one shot and there goes your arm a few hundred yards behind you.
OK, thats a good Argument and I'm on "your side "
( but it does NOT concern the Size ! A Krime Cannon will still fit into a Troll Cyberarm

)
I myself would never do such a nonsense ( a Railgun or Assault Cannon in a Cyberarm )
Those Guns are for rare and Special Situations.
And I need the Slots in "my" Cyberarms for other Gimmicks (like Cyberarmor, Monowire, intern Comlink, hidden Secret Compartments for Smuggling, etc)
When the Moment comes and I need a Railgun I get it out of the Weaponslocker or hidden Bunker and then "Rock & Roll" !
But I would never run around with it so that the Cops can find out about it and arrest my Char
(this would be extremely Stupid ImO)
without a stupid Dance
Medicineman