One of the artists over at Splash Damage has painstakingly crafted the Resistance's Gerund rifle from Bethesda's Brink. Having started off with the stock Nerf Stampede ECS18, which artist Laurens Corjin purchased for ~$30, he saw potential for a working replica of the Brink Gerund (granted, of course, not quite as deadly as the original... we hope).
The artistry is phenomenal and extremely accurate to the game, but we'll let you decide. Compare the above image (the gun made by Laurens) to the original game's gun below:
Incredible, right? Laurens has an extensive creation log posted here, which goes into detail about the materials, the process, painting, design, and original ideas. The worklog also contains dozens of pictures throughout the entire process, so if you're a DIY type of person, you may gain some insight from his procedures. Laurens says the gun took him six weeks of on-and-off work to complete and cost him around ~$230 to finalize. Given the quality of his craftsmanship, it was totally worth it.
Besides, Bethesda gave him a shout and they noted that they'd like to see more user-created content, so if you have custom work you've done for their games, email them!