Rail guns have a significant issue (which doesn't appear to have been overcome yet[1], unless they're just playing misdirection) that's fairly fundamental: the gun wastes a lot of energy on tearing itself apart. The rails would much prefer to crash into each other spectacularly than to send the projectile anywhere, and you have metal turning to plasma everywhere there is contact.

It seems fairly feasible to fix the energy usage by just throwing more energy at it (and the Zumwalt demonstrates that capacity is relatively portable), but the wear on these things is going to be a massive cost even if they reduce the problem 10x from where it is now.

[1]: http://www.popularmechanics.com/military/weapons/a21174/navy...

>can use any lump of feromagnetic material and it gets so mich kinetic energy that no explosives are needed? If yes, then those bullets should be really really cheap.

it isn't explosives that costs money. A typical $3K "dumb" bomb is made "smart" by bolting-on $25K JDAM guidance kit. When Lockheed Martin is done with that "lump of feromagnetic material" you'll wish it was made of gold instead :)

No, the rail-gun projectile will be a fairly sophisticated thing. It has to have a guidance system to be able to hit things at long range, so it includes a lot of electronics. (Similar, in fact, to the projectile in the article we are commenting on now.)

But the projectile is not the most tricky part. The plan is to develop a single "hypervelocity projectile" which can be fired by both chemical and rail guns, so it seems likely that that part will work out.

The rail gun itself needs a lot more research and development to get to a usable form. See e.g. this rather pessimistic article[0]. Meanwhile, the Navy doesn't care enough about the Zuwalt guns to even buy ammunition for them. How much will they spend on building a better gun?

[0] http://www.defenseone.com/technology/2016/06/can-navys-elect...

Heard a comment from a recently deployed logistics guy: "A great projectile for a rail gun would be a used-up double-A battery - we always have plenty of those."