Just as a guess, I'd say the bullets had a lead core and the magnetism was in the jacket. That's because steel is only about 70% as dense as lead and if the entire 230gr bullet (except for the jacketing) was steel it would need to be about 30% larger in volume. But, I suppose, that extra 30% could be on the back of the bullet and down inside the case.
Miller...er.... bullet puller & hacksaw time!
When you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind. [Lord Kelvin]