Hello,
First Person Scale is the method to prevent First Person Primitives from overlapping the world. Keep in mind that you can scale meshes to be smaller than the camera’s near clip plane, which will cause them to be cut off.
For your test project, a value around 0.5 was able to prevent clipping, while keeping the entire rifle visible.
For more information, please see this ticket:
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Note that Unreal Engine 5.5 has a bug related to First Person Rendering, which might also be affecting your use-case:
Please let us know if this helps.