I'd lay money on those tubs holding 20mms - or even 50cal MGs, now that I think of it - because those ships couldn't take the top-weight of (just) twin-40's, or even single-3". (Remember, the main armament was only single-4"!)

Prove me wrong and win a free 4-pak of quad-40s - or twin-3" - either of which you can chop apart, for whatever your Bainbridge would need...

Cheers,
-Matty