The Staghound .50 (12.7mm) cal can hurt the back/top of a Stug3g. BUT...
20mm AA and 20mm Sdkfz/Ostwind shots do not. Vierling .50 cal shots do not. 40mm allied AA shots do not. 23mm IL-2 cannons do not.
I don't know what's up with that, but I tested it out today.
hmm interesting..any words from the devs on this one? :)
Correction, 23mm IL-2 cannons DO, but they do no damage.
EDIT: It appears the Vierling is a 20mm gun. It still dosen't damage the back/top of the Stug3g though.