I tried some stuff and found out maybe its something about the physicAsset. I removed it from the SkeletalMesh of the weapon and the weapon was visible again. But without a physicAsset no forces like gravity get applied and weapons floating in thin air after dropping them.
I check the physicsAssets with a complete other item where everything works fine but I coudnt spot a difference all options are the same. I also tried creating a new PhysicAsset directly from the mesh in the context browser but unfortunately the weapons where still invisible after attaching.