[SG2] GMS/P against infantry?

Another interesting event in last weekend's game was a fluke shot
against my
PA squad.  One of the defending FSE infantry suqads fired a GMS/P
against my
PA squad.

It was unclear if this was allowed or not, but we assumed it was
appropriate
and rolled against the range, modified by cover.  The missile managed to
hit
and (due to good rolls) cause not one but TWO casualties.

Was this correct?  Should we have used the ruling for heavy weapons
against
infantry (d8 impact for shrapnel as opposed to the d12 of the missile)
instead?  

Tom