Rubin's pieces often use photos as their basis. I was told these pieces were gouache on on vintage Chinese magazines. You can see that (after you are told at least!). You can see more of Rubin's work at the gallery website.
Völker's gun pieces are oil on canvas. At first glance they may remind you of some pieces by Andy Warhol. The angle you see these weapons at is different from the Warhol guns; they give you a feeling they've just been used or are about to be used.