Yep. "at the start of that fire phase". My original question was about what happens if the guarding unit becomes berserk during its MPh.
Well, think a bit Hollywood:
We know from the rules that the Zerkers will kill the Prisoners 'as if using a SW'. They can't do that
during the MPh.
Zerkers will still not be so kind as to let the prisoners run away just to charge others. Illogical. So what happens?
The Berserkers will use the Prisoners as a 'Human Shield', i.e. they have to participate in the charge that has been triggered by the Guards becoming Berserk during their MPh. During the Charge, the Zerkers will likely get shot at - along with their 'Human Shields' which they drive before them. If the Zerkers are capable of entering an enemy-occupied hex (and they and the Prisoners survive all this), then they can fire TBPF/halved during their AFPh (not yet in Melee) and at this point, the 'Human Shields' - being of no use to cover the approach any longer - are massacred 'as if using a SW', i.e. the same way as the Berserkers could use a LMG they carry during the charge.
If only a Zerker-
HS Guard with Prisoners survives, then it has to decide to either 'use a SW' (i.e. massacring the Prisoners) or firing their Inherent FP vs. the other enemy units in the same hex. Likely, they will select the 'massacre' as being a Guard in CC is not favorable as a HS and also make CC sequential in favor of the Prisoners.
I think this is a pretty solid solution to address all possible situations that would work within the general logic of the rules (even if not explicitely covered)
and also makes sense based on 'real-life' - erm - 'arguments'.
von Marwitz