Whilst doing nothing of the slightest use to humanity, I came up with Green-er combat.
To whit, a rotary weapon with two sets of contra-rotating barrels (think two Gatling guns side by side, whose gears interlock).
With a regular rotary weapon the cartridge goes into the chamber, gets fired,
and the cartridge cases go all over the place. The clever bit is, the brass is collected, passed in a wide loop around the weapon giving the slipstream time to cool down the cases.
The cases are then re-filled with powder, and inserted into Gatling gun no. 2 which has a set of long roads to force the new bullet backwards down the barrel (of course this is much easier with adjustable rifling turned to zero, just running parallel to the axis of the barrel)into the refilled case along with a new primer.
One additional advantage is while gun 1 is firing, gun 2 is a having a bit of a rest, so allowing the barrels to cool down.
Of course it would be more green to also collect in the used bullets as well, but tests with elastic or fishing line proved...shall we say problematical at best, in particular with targets who have already been buried by the survivors. Just don't ask, really.
And so a great deal of copper and tin gets recycled at the point of use.
Is it just me or do I seem to be channeling His Borgness these days? It all means something.