To be perfectly honest all these AR-15 variants are pretty hard to identify because they basically all look the same, especially now since everyone loves the modular system. This looks most visually similar to the (relatively) new ROK STC-16 carbine but it has a different stock and more importantly for the user, a safety switch at the top of the grip. In the end, it's probably some custom tricked out AR-15 SBR version with a custom everything so the artist doesn't have to pour over every detail of an already existing rifle to get model consistency.