Do not click on the gun, but on the Shell icons. If this not work, open in notepad Basic Cfg file located in the game data/cfg folder, and find the line:
[AMMO_TYPE0];10.5cm SK C/32
Idx=0
Year=1939
Month=1
AP=30
HE=40
SS=60
Renown=0
I take this from my modifed CFG file. Maybe You have other values in yours, it's normal.
Change [check] AP or HE values, [SS - Light shell]. If you use many mods you need to check them first, and copy files before any changes.