This sight is designed to fit on Rival series Nerf blasters (and their proprietary rail).

There are two versions of this model: with and without a "split".

Version 1: "No-Split" This version slides over the end of the Rival rail. This version may or may not require the M3 hardware to secure the optic to the rail (the friction fit may be enough for you)

Version 2: "Split" Some Nerf blasters (like the Helios pictured) are designed in a way the prevents this optic from being slid onto the end of the rail. This version is designed with a "split" in the base that allows the optic to be flexed open and mounted onto the rail. This version will require M3 hardware (bolt and nut) to secure it to the rail.