you have to reach a certain velocity to have jacket seperation, so you were more than likely under this velocity with the 40g as for the 75,
And you would also see seperation take place, in a cloud like puff,
they may have been going fast enough to stabilise, to you know when a bullet has not stabilsed as it will yaw to the target leaving an odd shaped hole not not a circular one like a bullet should......
snap.