Some thoughts on copper bullets.
As already pointed out, we have all copper hunting bullets from a couple of manufacturers, so this is not new technology.
One of the problems with replacing a lead core bullet with a copper bullet of the same weight is that sectional density is lower, meaning an all copper bullet is longer than it's lead analog. This in turn means that copper bullets of the same mass require a faster rate of twist compared to lead bullets.
Second, while expansion is desirable in a hunting round, it is not always the case with a military round. Barrier penetration is an issue that hunters don;t have to deal with. One of the reasons that ball continues to be used for military applications is that it is an all around bullet - it works when shooting bad guy, bad guys with light armor, vehicles, barriers, etc. Military ammo has to do a lot more than just produce a casualty.
Finally, someone brought up MRX. This is not an all copper round. It has a tungsten insert to increase sectional density, and is thus shorter than an all copper bullet of the same mass. It has a higher BC than all copper bullets.
A military bullet derived from something like MRX makes sense in that existing barrels could be used, and a properly formed tungsten insert would make a decent anti-armor, anti-barrier projectile.
Whatever the case, a copper bullet is going to be significantly more expensive than the current M855. Anyone who has bought TSX knows this.
Unfortunately, the military seems fixated on copper as a 'green' solution. There are some very promising sintered metal and polymer metal matrix projectiles that could replace the classic copper guilded lead core bullet at less cost and with better terminal performance.