After further testing I was able to get reproduce the clipping in the first video in your last response. If you use Set Collision Object Type on the magazine so that its default is physics body and that it only gets the new collision type after being attached (IgnoreGun) This will allow the gun to collide with the magazines on the table that are not attached to the gun. You will then need to set the collision object type back to physics body when the magazine is removed from the gun (example below).
Example:
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As for the clipping through static objects (when the gun is placed up right on the table and the magazine clips through), this is a known issue that has been submitted to the developers for further consideration. I have provided a link below to the public tracker. Please feel free to use the link provided for future updates.
Link: Unreal Engine Issues and Bug Tracker (UE-30895)
Make it a great day