This is a suppressor-style barrel extension for the Nerf Stryfe (and presumably other blasters with similar accessory points). There are no moving parts or springs, but the fit is very snug and there are several raised lips that lock the piece in place. It can be easily added and removed by hand without fear of it falling off.
The internal diameter matches that of the "outer barrel" of the Stryfe. At ~25mm, it's much larger than the darts and doesn't impact performance, but my 7yo will swear it's way more accurate.
I printed it upright (just like the STL) with a brim, but the brim probably wasn't necessary. The steepest angles are only 45 degrees, so no supports are necessary.
I knocked off the sharp edge from the brim, but otherwise nothing.