One thing I really like with the miracle rules in godlike is that there are often several potential powers that can create the same result with enough extras/flaws. There are IMNHO very few miracle ideas that cannot be done using the existing powers in one way or another. This can be a bit unbalanced according to point cost though.
The magnetic power is a ver cool one indeed. I suppose you could do something similar with the control miracle but that miracle is not very concrete so it can be hard to understand what wouuld really happen if you alter this or that. It's a game of superpowers though so you as a GM can always decide what the effects will be despite every physical law that it will break which is a very good thing.
I happens to like physics so here are some comments.
-for example, when a rifle a couple of yards next to a tank is affected with the power
the tank will not be affected (unless your body is really really high) and the rifle remains
where it is; but if the tank is affected, the rifle flings to the tank
Acctually both the rifle and the tank are attracted to each other by the same force wheter you magnetize the tank or the rifle and the effect will be the same -the rifle will fling to the tank (since the force is to small to move the tank). Think of the tank and the rifle as connected with a a rubber band. It doesnt matter if you move the rifle away from the tank or drive tank away while holding the rifle as a measure to tension the ruber band. As soon as you let go of the rifle ite will fling to the tank and the tank won't move.
I am no expert on bullets but... Bullets can be made om many different materials but rarely if ever of solid copper. Well they can be made of copper but that would be very expensive and I doubt they will be any good and definitively not better than ceaper variants. What you want is a heavy bullet beause it will have more kinetic energy and wont loose energy as fast as a small one giving it longer range and better penetration. Often you design the bullets with a core that is coated in a jacket. The jacket are often made of sof copper-alloys to prevent barrel wear so they might look like they are made of solid copper but they are not. And some bullets were acctually made of magnetic metalls - steel and iron.
Here is a list of common metalls used in the core of small arms bullets.
Lead is a high density metal and it is soft and have a low melting temperature and has therefore been the choise of bulletmakers for hundreds of years and is still widely used despite som negative environmental effects. Since lead is soft it is not very good at penetrating hard material such as armor but works well against flesh. The .30-06 Ball is a bullet with a lead core.
Against armor you want a very hevy, very hard bullet. The ideal choice would be tungsten but that is very expensive and rare and difficult to form in the manufacturing process. There where armored piercing tungsten rounds available during WWII though. An example of tungsten ammuntion is the German 7.92 mm S.m.K.H (Spitzgeshoss mit Kern, gehertet. A cheaper but less effective variant is to make the core of hardened steel, sometimes with a lead tip. The .30-06 AP is one example of steel core ammunition, another is 7.92 mm S.m.K. The soft copper jacket around the hard core is needed to prevent excessive barrel wear.
Since many metalls and a lot of them are needed in a war economy many of them might become scarce. That was especially the case in Germany. Therefore ersatz materials were used. I believe that is the reason why the Germans developed the 7.92 mm S.m.E Spitzgeschoss mit Eisenkern (Eisen = iron). It had a soft iron core.