The images of the colored version are very similar, except one is without extra, dramatic shading. That one has lighting and shading involved, but is largely intended to be rendered in it's local color.
As far as boltguns go in warhammer 40,000, the types and versions used by the Chaos and Heretic factions are many and varied in pattern, accessory, size, etc. The one pictured here has more ornate touches with gold and brass work, inscriptions, a strap made of the tanned hide of some fallen xenos, the dried hide of a human foe, and warp-tainted metal.
More than likely, based on the color scheme here, this chaos space marine bolter is the property of a champion of the Word Bearers chapter of heretic Astartes.
Hope you all enjoy. See below for more fan art of Warhammer 40K: Eternal Crusade.