I'm having the same issue. On my previous patrol I was able to assign crew to the deck gun. The patrol was sucessful and I got a medal and several crew were able to be promoted. I started the new patrol and I have no slots to assign crew to the deck gun. :doh: I can use it manually tho. :)

Let me ask this...did you remove the old gun from the stern before installing the new one on the bow?
If not the game gets confused and thinks your trying to have 2 guns installed both stern and bow, according to your screen shot this would seem to be the case.
A Porpoise class sub can't have 2 deck guns installed.
According to the upgrades.upc only 5.25's can be mounted at the same time and only after 01-45

Adding entries to the upc file to allow double deck guns would be easy (although not realistic)

Anyway something to think about :D

*edit* well my idea was wrong, at least in part, old guns aren't uninstalled we simply drag the new one over the old...

any fix to this yet?

I found this in the UPC file:

__________________________________________________ _________

[UserPlayerUnit 1.Compartment 7]
CompartmentType= 2
StatusActive= No
ID= GarGunS
NameDisplayable= Aft Gun Deck
Type=NULL
FunctionalType= ArtilleryRoom
MechanicalCoef= 0.5
ElectricsCoef= 0.5
GunsCoef= 0.2 ;0..1
WatchmanCoef= 0.5
WatchStandingCoef= 0.048
MaintenanceCoef= 0.0144
RepairsCoef= 0.072
ReloadingweaponCoef= 0.072
SleepCoef= -0.12
LeadersSlots=0
CrewMembersSlots= 0
EffciencyDenominator=2
EffciencyDenominatorBS=2
Hitpoints=200
CrewExposure=0.7
EquipmentsExposure=0.1
WeaponsExposure=0.1
ExternalDamageZoneTypeID3D= 198
DamageDescription1= NULL, 0, 0.2, 0, 1, 1, Minor damage, 0, 0, NULL, 0, 0.2, 0.2
DamageDescription2= NULL, 0.2, 0.6, 0, 1, 1, Medium damage, 0, 0, NULL, 0, 0.2, 0.5
DamageDescription3= NULL, 0.6, 1, 0, 1, 1, Heavy damage, 0, 0, NULL, 0, 0.2, 1
[UserPlayerUnit 1.Compartment 8]
CompartmentType= 2
StatusActive= No
ID= GarGunB
NameDisplayable= Fore Gun Deck
Type=NULL
FunctionalType= ArtilleryRoom
MechanicalCoef= 0.5
ElectricsCoef= 0.5
GunsCoef= 0.2 ;0..1
WatchmanCoef= 0.5
WatchStandingCoef= 0.048
MaintenanceCoef= 0.0144
RepairsCoef= 0.072
ReloadingweaponCoef= 0.072
SleepCoef= -0.12
LeadersSlots=0
CrewMembersSlots= 0
EffciencyDenominator=2
EffciencyDenominatorBS=2
Hitpoints=200
CrewExposure=0.7
EquipmentsExposure=0.1
WeaponsExposure=0.1
ExternalDamageZoneTypeID3D= 43
DamageDescription1= NULL, 0, 0.2, 0, 1, 1, Minor damage, 0, 0, NULL, 0, 0.2, 0.2
DamageDescription2= NULL, 0.2, 0.6, 0, 1, 1, Medium damage, 0, 0, NULL, 0, 0.2, 0.5
DamageDescription3= NULL, 0.6, 1,

__________________________________________________ _________________

What is this for? Can you manipulate this to tell the game your gun is FWD or Aft? Check out the highlighted.