This prop was used for my Nyota Uhura cosplay and was originally white/neon orange.
The following was done to get the final silver finish:
1.) Prime the gun with 2-3 layers of Rustoleum Black Gloss spray paint
2.) Cover with 2 layers of Valspar metallic spray paint
3.) Paint the tip red and the inner ridges black
4.) Cover with a gloss clear coat
It's simple and easy! This method basically works with any metal prop (: