This is a tri-rail mount system for everyone's favorite bug killing assault weapon. The inside has a tapered fit on the smaller bore but you can add your favorite adhesive to keep it more secure if necessary. Designed to mil-spec per the link in the design description
I printed this without a brim/rafts at first and had some minor adhesion issues with ABS.
Remove the supports/brim and lightly file inside corners of rail for best fit of accessories.