Weapon cutomization and gun animations

How do you add a weapon customization with things like the cylinder in a revolver while retaining the gun animations?

Make your weapon in your 3D software as a skeletal mesh, rig the the customization parts with the same pivot as the main weapon to the weapon rig. First Import the main Weapon with no Rig, Name the new created rig clearly
. Then Import the parts with the new created UE4 rig and use a blueprint the master pose component
or sockets.