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This is about 8 years late, but if you have the mod "UltimateWeapons for JK2", you need to :
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1. Open the .pk3 file "UltimateWeapons_JK2.pk3" (either via 7-Zip/WinRar or rename the .pk3 to a .zip and open it normally)
2. Go to the folder "models/players/kyle" (inside the 'kyle' folder is the file "model_fpls2.skin")
3. Delete the file "model_fpls2.skin"
Deleting that allowed the first person lightsaber mode to work as intended.