Sometimes on a few systems UT weapon meshes can completely or partially disappear! 
To fix this You can open Unreal Editor and in the Actor Class browser open the referring 
U-file of the disappearing weapon and with leftclick open its default properties. 
There under Display find the line LODBias and increase it from 1 to idk .. 3 or 4. 
Then make sure to have the ShowPackages box checked and mark the edited mod file and 
then again under the Actor Class browsers options do save it. 
Now the weapon mesh ingame should not become invisible anymore! 