HK Straightening Mandrel - G3 & MP5

thingiverse

This is a billet mandrel I designed to straighten HK G3 and HK MP5 receivers. The designs are different between the two - this is not like the original "universal" mandrels which chose a compatible design between the two receiver internal shapes that was really correct for neither. The MP5 design is based on 3D scanned MP5 receivers and checked against alleged HK drawings that are currently floating around. The G3 design is based loosely on information I received from holescreek with regard to his original straightening mandrel design and extensive prototyping, measuring and testing with three different G3 kits and receivers of various makes. The result is a mandrel design that will work to accurately correct dimensions on virtually any G3 or MP5 receiver, regardless of how warped or otherwise out of spec they may be. Note: the final cuts on the G3 receiver were taken with a 3/32 ball mill to mill out the junction between the larger 1.52" bottom circle and the smaller 20mm top circle. This gave the best ID/OD for stock and bolt carrier fitment. You will want to include this feature on your 3D print, which is otherwise not shown on the STL because it's a dimension added as a result of the manufacturing process, not the CAD. A 3/32 radius instead of a sharp corner there should solve the issue. If you're doing it as a two-piece design, you could ignore the radius without issue (probably). I've provided the files as STLs to use as a basis for your choice of manufacturing method. I CNC milled my designs from a billet of 1018, but manual milling is certainly possible with enough patience and correct taper and radius mills. A two part design would probably be desirable if produced with manual machining. 3D printing may be possible, and I’d really be interested if someone could make this design work with 3D prints. Included files G3 center mandrel - hammered into the center of the G3 receiver in order to form the ID G3 side wings - pressed into the grooves on the outside of the receiver in order to form OD MP5 center mandrel MP5 side wings Universal bottom clamp - clamps along bottom in order to prevent bulging of receiver when reforming rails. In my design, they attach via screws to the central mandrel. Your milage may vary.