So I found this
AK47.net thread/article on the M231 (aka Bradley Port Firing Weapon). This puppy was apparently much beloved by the T2k guys, so I figured I should post it here!
From the linked thread, it seems to be an abject bullet hose, firing too fast to malfunction (OK that's a bit tongue-in-cheek), and of limited utility. OP in that thread suggests it could perhaps be useful for armor crewmen, maybe, but that comparable if not better options exist.
My impression from reading the big yellow book was that they assumed there'd be tons of these in use all over the place for militia, second-line units, etc. Makes sense, they apparently weren't all that useful in their installed role; in Europe, there's quite a dearth of firearms; in the States, it's a full-auto weapon of reasonably familiar manual of arms.