Military switching to "green" bullets (Military.com)

[stanc]

The concerns was that making the core out of copper would make the rounds too expensive. Wonder why not steel?

Since steel has lower density than copper, using a steel slug would make the bullet longer, and thereby eat up more powder space.

The other question would be if the 6.5 Grendel would not be acceptable then what would be acceptable?

That's the $64,000,000 question. Nobody knows for sure.

It is clear that we are not going back to the 7.62x51 and that the 5.56x45 is just too light for many missions. While I think that the 6.5mm would be the ideal caliber, some might see that something closer to 7mm would be better.

Then is what the dimenisions of the case should be. It appears that it should be close to 10.7mm in diameter. And no shorter than 40mm but not much longer than 45mm.

Bullet and case dimensions should be whatever they need to be to achieve the goal. So far, IMO there is insufficient test data publicly available to make a rational choice of bullet caliber and weight.