Threaded Barrel Insert for 'Hi-Capa Baba Yaga'. It uses 14mm left turn threads that'll fit most tracer units. It's a press fit so that it doesn't just fall out, but that being said, drilling part of the barrel and adding a retaining screw will greatly help( i haven't done that though). You need a well-calibrated printer for these threads; I used the Prusa Mk3S+. These inserts do eventually fall out, so I recommend printing a couple extra, as once you've pressed one in, the plastic deforms and they can't be reused.