Ductile Iron Bullets why not?
 

Something I have long wondered is why are soft iron/steel bullets illegal?

It’s not like everything has to be armor piercing and the soft iron is cheaper than lead or copper

The “mass” argument doesn’t hold water for me as a bullets range is determined by the amount of powder, the barrel and the frontal area of the bullet, if mass is an issue make the bullet longer and it should still fly straight and hit with the same force.

Soft iron and even steel can easily be made to smoosh up on contact similar to lead.

And if we really want it to smoosh out an aluminum or hollow :0 tip would do that also

If we are stuck with the cost/utility problem forever why don’t we just solve it?

Under the armor piercing ammunition "cop killer" law, iron bullets would only be illegal if designed for a pistol. Just about any rifle round will penetrate soft body armor so they are exempt from 18 USC 921(a)(17).

So you can legally make soft iron rifle rounds. You probably wouldn't want to because iron is not as dense as lead and other common bullet materials. Lightweight bullets have higher initial velocities, but shed that velocity quickly due to aerodynamic drag. Rifle rounds are generally intended to shoot targets far away and a lightweight bullet that had shed a lot of it's speed won't carry as much energy as a heavier bullet with a slower initial velocity.

You also have the problem that the rate of twist of the rifling works well on only a small range of bullet weights. Too light or too heavy and the rifling won't stabilize the round as much - think of a football thrown with subpar spiraling - which hurts both accuracy & precision. An old 30.06 rifle will probably best shoot 175gr bullets, but if you're in a lead-free state your probably shooting 165gr copper bullets. That's pretty close, but still people notice a decline in accuracy. And copper is much more dense than iron. You can't just keep making the bullet longer - at some point it will no longer fit in magazines/chambers or leave enough room for powder.

You can overcome this by designing the rifle from the beginning to shoot lead free ammunition, in which case iron's biggest downside is that it would have to go much faster to maintain the same energy at a given distance. And shooting bullets faster wears out the rifling quicker. Lighter bullets are also affected more by winds.

Could you then design both a new rifle and new ammunition - an iron round that was much longer than normal for lead and a rifle that would chamber it? Probably. I bet that would work. Could you get enough people to adopt it to become commercially viable?

If the answer is anywhere, it's probably www.youtube.com/c/taofledermaus. They specialize in 12ga rounds, but cover a wide variety of shapes and materials.

For instance, 4/0 Copper Cable Flechette Rounds - Tested

You end up with low sectional density and high ballistic coefficient bullets. They will be more difficult to stabilize.
There are more and more solid copper options and solid copper is usually a drop in replacement for copper and lead bullets.
I have found the solid copper bullets shoot great, only disadvantage is they cost at least twice as much as lead. That's just the bullets, everything else costs the same.
I use solid copper as my brush bullets as I might blast a mule deer from a few meters away in stand of brush and trees. A solid copper bullet won't fill the deer with pulverized lead particles. I actually was going to switch over to solid copper but I can't reliably find those copper bullets.
For longer range like 200 meters and up I use lead bullets. After a brief flight and a little aerodynamic heating the bullets slow down enough to not uncontrollably fragment.

Thinking more about it, I don't think you can cast iron to the same tolerances so I suspect the manufacturing costs would be higher to get acceptable performance.

https://en.wikipedia.org/wiki/Mini%C3%A9_ball#Designs
Iron won't obturate. {I wonder if they used pork grease?]

Such as in that Jihawg Ammo brand, which claims to have some pork grease?

Iron bullets would have to be machined and slipped into a copper jacket. Solid Copper bullets are both, they are stamp into rough shape and machined into final shape.
Copper and lead bullets are just brute forced into shape with minimal to no machine work to finish them.
Lead bullets are brute forced into shape or cast.

Makes me wonder how much more expensive iron or steel bullets would be compared to those low-lethality elastomeric bullets used for riot control.

Why does it have to be ductile cast iron? Scrap auto bodies melted into rod or actual bullets. Shouldn't care about the exact physicals.

Didn't the AR15 tumble back during 'Nam? Iirc, your friend with the BAR was handy to have for tree snipers.