You must also change the ammo slots to represent match the number of shells you are going to add.
In a Type IX I carry 250HE......Go to:C:\Program Files\Ubisoft\GWX2.1\data\Cfg
Open up the Basic CFG file and scroll down to

example is for an IX to 250 rounds)
[SUBMARINE_AMMOLIMITS1] ;105mm
MaxAmmoPerSlot=110
[SUBMARINE_AMMOLIMITS2] ;88mm
MaxAmmoPerSlot=110
[SUBMARINE_AMMOLIMITS3] ;37mm
MaxAmmoPerSlot=400
[SUBMARINE_AMMOLIMITS4] ;20mm
;MaxAmmoPerSlot=1000
MaxAmmoPerSlot=2000
Change the 105mm from 110 to 250
Now scroll down to:
[AMMO_TYPE0];10.5cm SK C/32
Idx=0
Year=1939
Month=1
;AP=40
;HE=60
;SS=20
AP=0
HE=110
SS=0
Renown=0
Change both HE values to 250
Good Hunting