"You enter a building looking for supplies and find a group of raiders, after an intense shootout you look down to loot the bodies only to find that there are no spent casings on the ground"

This mod does the same as Shells Rain for Fallout 3 and Permanent Weapon Shells for New Vegas.
It increases the duration and amount of shells when firing a gun.

Values changed:

fGunShellLifetime: 600
Shells will now be visible for 600 seconds (10 minutes) before disappearing. In vanilla they last 5 seconds.

fGunShellCameraDistance: 51200
In vanilla it's 512, increased x100 times. It may seem a lot but vanilla 512 only shows shells from enemies if you are close, this change shows them from a distance.

iDebrisMaxCount: 1500
Max number of visible shells is increased to 1500. In vanilla it's 25.