Well, I'd say such small difference is not really important in FH2 and not really in real life either.
In real life, the bullet would either hit something critical or it would be useless. And when it goes through commanders face or through engine block, I don't really think it matters how big the bullet is. I find it hard to believe, that neither of these guns could cause armour to spall so much it would be lethal to anyone inside tank.