HK MP5 Receiver Straightening Mandrel

Update 9OCT2026: I continue to update these as I discover refinements. These updated models can be found on my site at https://marbleduck.com/articles/mandrel.html

Several years ago, I machined a run of billet HK receiver mandrels. I still get the occasional query for them; unfortunately, I sold my shop and resigned my FFL/SOT when I went active duty and can no longer produce these. At the time, I posted a half-assed low-res STL of the models I built which admittedly wasn't very helpful. With the rise of overseas/domestic CNC machining resources that are now available to the consumer, I figured it was time to publish proper drawings. Also, it seems that someone has appropriated my original work for commercial use without bothering to inform me. I'm sure you can find these online. How annoying.

Anyhow, these resources can be used to order your own mandrels direct-from-manufacturer for $600-800. The tolerances are built on my best guess from a half dozen samples, 3D scanning, and a whole lot of trial and error. I welcome any input from those more experienced folks out there! I would also like to again extend thanks to holescreek, who provided the basis of information for me to get started in building HKs/their tooling and a few of the fundamental dimensions on the G3 mandrel.