TL/DR:
This looks like not animated asset, get it into 3d app like blender and animate there. Can be done in unreal and trough blueprints, but its more work. Or get already animated weapon asset from marketplace.
Longer explanation:
Find base mesh, see if it has sockets, snap to sockets.
Or
Create Blueprint, add all those meshes there, use hierarchy tree (to the left where all components are shown in tree) to add them all on top of one another.
However it all kind of depends on how that asset was done. Also they all seem to be static meshes, so for animation like replacing ammo etc, you need either animate it all yourself, or make it trough code in blueprints.
What gun it is? there may be much better (animated guns) on unreal marketplace. Sometimes some guns are for free, or quite cheap. So either spend time working on this gun or spend some $$$ in marketplace.