What you have there is a mock up based on a Heckler & Kock G-36. Here it is in a couple of variants.
As you can see, your video game version has the long barrel, but also has the longer optics rail that is raised above the weapon. The bull barrel end is no doubt to simulate a suppressor of some sort that will thread on and off depending on mission type.
The stock has been bastardized to be more in line of some hybrid offspring from the Wa-2000 and the PSG-1.
You aren't going to find one like that, you are going to have to buy an airsoft model and do the mods yourself. Is your costume worth another $500-$600 dollars?