Nerf Zombiestrike Nailbiter. Looks to be double action like the Voidcastor with a clip that I would assume ratchets up with every shot. Credits to Alpaca XJ : r/Nerf
Nerf Zombie Nailbiter 6KG Modification Upgrade Spring Coil | Etsy
Nailbiter - HPA Edition : r/Nerf
My DeWALT Nailbiter with Zoom and Doom pack. Reposted with orange tips. : r/ Nerf
NERF Nailbiter Mod Guide (Internals, Spring Upgrade) | Walcom S7 - YouTube
NERF Nailbiter Zombie Strike Toy Blaster – 8 Official Zombie Strike Elite Darts, 8-Dart Indexing Clip – Survival System – for Kids, Teens, Adults : Toys & Games
Nailbiter internals : r/Nerf
Nerf Zombie Strike Survival System Nailbiter Blaster | Nerf