This is a replacement muzzle assembly for the Adventure Force Commandfire that allows you to use any Nerf designed ( and presuambly 3rd party ) muzzle attachment to your Commandfire. Alternately, it shortens up the barrel nicely.
This piece incorporates a modified version of the muzzle lug from the Meaker MK 15 by Meaker VI ( https://www.thingiverse.com/thing:2957944 )