This is a top picatinny rail for a Nerf Retaliator or Recon with the upper priming handle removed. You will need to use the pin that jam door uses and you will need some M4 nuts and bolts. There front end of the blaster tapers downward away from the rail so there may be a small gap there. The rear is designed to secure into a stock. Compatible stock links to follow.
The top rail can be printed without supports but the mounting play may need them.
I used 0.2 layer height with 3 outer wall layers for a total wall thickness of 1.2 mm. 20% infill.
Retaliator Stock: https://www.thingiverse.com/thing:3065305