You think too much realistically 
As a devloper you are director and magicsion, it does not matter how you do it things, all it matters is what player see. Does player see the bullet with case in weapon? If not then whats the point of separateing something that they don’t see, just spawn bullet from front of gun and case on the side (that could be particle). Even if they see bullet in gun you can make it animation in part of what they can see and hide it somewhere and still spawn buttlet and case.
Note that lot of game don’t even do bullets and just do line tracing, because normaly human don’t see bullet traveling in the air as it simply too fast, or else they want to make it realistic or do some slomo mechanics