S&T TAR-21 Hop-up Adapter for M4 Rotary Hop-up

thingiverse

The hop-up in question is a generic and commonly available metal rotary hop-up, mainly for M4s. The issue is that the mag feeder part does not reach down enough to unlatch the mag BB stopper. The "TAR-Hop-up-adapter-feeder.stl" file contains the tube adaptor part. A secondary issue is that it does not have the top pin to hold the outer barrel. That is what the "TAR-Hop-up-retainer.stl" file is for. It is a tight fit, but it should be just the appropriate size to match with the screws that hold the outer barrel. This should be printed on the side, as shown in the image, to reduce the chance of cracking along the layer boundaries. I recommend PETG or similar material, especially for the retainer, PLA has a tendency to warp over time under stress, and the thin section may break.