Version 3 of the H&K MP5 Magazine loading little .177 Cal (4.5mm) BB's sucks, even with the good magazines that Umarex provides with the H&K MP5 K-PDW. This is a speed-loader for the magazines to drop those 40 little balls into the magazine.
Print at 100% in the orientation provided 3-ish Hrs on Ender 3 High Quality, PLA, clean out the support and ream out the BB hole to ensure easy BB passage and no plastic in your magazine.
Pull Magazine Follower back until locks, place on top of magazine, should be snug. This holds the retaining latch back so the BBs should just fall into the magazine. Lightly shake if you get a BB Jam at the hole. I may make one that has a 40 BB Measure/larger hopper, but as is this cuts loading time down to about 10-20 seconds.
Designed in TinkerCad. You can Tinker it yourself https://www.tinkercad.com/things/63p5vOr6GbR