Hello! I want to share with you the way I paint the bolters, bolt pistols, etc. for the space marines. I use always the same tecnique so you can use it for all the marine legions/chapters if you want.
I start with a the black basecoat, then I highlight the edges with vallejo neutral grey (citadel dawnstone works too).
For the second highlight I add white to the neutral grey 40/60 and I highlight the edges again but a smaller area. For the grip on the tigrus bolters I use a basecoat of vallejo german c. black brown and then highlight of citadel steel legion drab.
I hope you like it! It is quite simple but the bolters look very good! Happy painting!