It's the Surbaissê AP does sometimes do no damage?

I do wonder what gaijin has done to her APBC, because sometimes they pen and do no damage, sometimes they bounce randomly even at almost flat angle and sometimes the shell seems eaten by tracks or some random object on enemy tanks. I know that her AP is not good for angle target but please it’s kinda annoying for 23KG of steel to do no damage at all, it’s not always happen but when it do happen it’s annoying.

And also her gun is derivative from the American M103 120mm Gun right? Why she can use Heat-FS? And lastly is there any documented usage of another ammunition that being used during testing?