So primitive... Looks like there is no problem with amount of rng in game but the functions computer uses, that kind of function will never be even close to fair or real. With a straight line you can even penetrate a t-34 with mg-42 with 1,5% chance which is not so low considering it fires bursts.
I don't remember the specific number, but if chance of penetration is lower than it (2%? 5%? like I've said, I don't remember the exact number) it counts as 0%.
Saying that, MG42 incendiary rounds have +9 penetration, giving it 10.4 penetration at max range which should be enough to eventually kill a T-34 from the back (since 14.9% is most likely above the threshold for reduction to 0%).