I don't remember where it was, but I have seen a picture of austrian troops on erxercise 2 or 3 years ago. They really fitted some kind of small bag to their Steyr AUGs. Although the idea behind it was not polluting the environment, it seems to work with the AUG. If it works with it, why should it not work with other rifles?
If a party wants to collect the spent brass, I suggest the following:
1. You must have the time to search for the spent cartridges. If there is no time (enemies looking after you, or the party has to move on quickly), let the PCs grab a fistful of cartridges as an action.
2. I would suppose, that you will not find every spent cartridge (Unless you would really spent a lot of time with searching the ground underneath every bush in the area!). Therefore I assume, that 50% are lost, if you just pick up, what you find. If the party wants to spend some time after the firefight (Some minutes, depending on situation), let there still be a loss of 20 - 30%. One should also take in account, how the underground lools like: If the party did not move a lot and were firing from an ambush in a building, the picking up of cartridges would be a lot easier, compared to a firefight in some kind of wood.
If they were fighting in mud or swamp, don't let the spent cartridges be found
