The forward and rear firing mortars are not listed as fixed. Here is a simplified listing of weapon direct from the 3rd edition description:
Weapon systems are mounted as follows from bow to stern:
1x forward firing claymore mine
2x forward firing fixed M85C machinegun
2x forward firing 81mm mortar*
1x turret mounted 81 mm mortar*
2x turret mounting M85C machinegun and M10-8 flame gun
2x turret mounting Rh 202 20mm autocannon
1x heavy weapons pod A or B (choose one)
2x rear firing 81mm mortar*
2x rear firing M85C machinegun
1x rear firing claymore mine
4x left firing claymore mine
4x right firing claymore mine
* all mortars are breach loaded
I have always thought it was a typo, since the rear firing M85Cs are NOT fixed and the all the M85Cs were in some kind of limited arc, servo actuated pintle mount with an overlapping cone in the center. It is also possible they were trying to make something like this:
http://pcf45.com/misfire/81-50.html
My question was just to see if there were any real justification for a fixed mount on a ground vehicle.