Two types of Single, Double, Tripple and Drum Nerf mag holders.
Gravity held ones that you feed in the magazine from the top. These I use personally and have tested in play and did not have any issue with mags falling out even when running or jumping. However the only work with 18rd magazines.
Friction Fit ones that feed in from the bottom. Iv have tested these to make sure they hold magazine in but have not used them in play as I prefer my other version. These however hold Worker 22s on top of standard 18s.
The Drum mag holder feeds in from the top and holds the drum at an angle so that it does not bump into your body as you move/run.
Iv printed these holders with both the standard .4 nozzle and a .6 nozzle and the respective layer height for each nozzle. I didn't notice any difference in function. I do print using 3-5 wall line count depending on nozzle size.