HK G3/90 series Receiver Drilling Jig

Contact me/learn more: https://marbleduck.com/

This drilling jig, when paired with the below drill bushings and pins, can be used to accurately place the stock pin bushing, mag release, and paddle mag axle holes. It can also serve as a reference file to take dimensions from this part of the HK G3/MP5 receiver. Dimensions are taken from 3D scans and verified against the MP5 blueprints. Unlike other jigs, this jig accurately locates all holes based on the datums specified in the original prints: the dead center of the receiver rails, and the rear of the magwell stamping.

This design expects you to use a 3/16" pin for the paddle mag axle. If you'd like to use another size, swap out the bushing for your desired one and adjust the hole size on the linked Fusion file.

USAGE:
Once assembled with the correct bushings and locator pins, place the correspondingly-shaped rails into the rail indents on the outside of the HK receiver. Index the back of the magazine well against the square boss, as shown in the last picture. Clamp together, ensuring jig remains flat and parallel. Place in drilling machine of your choice. Drill.

LOCATOR PINS:
Using the vertical pin bosses and some 5/16" pins, you can accurately locate both halves of the jig together. 5/16" pins can be pressed into one half of the jig, and the other half of the jig can take the linear bearings (6391K165).

REAR STOCK PIN HOLES:
Press the .375 ID bushings (8491A726) into all four of these holes. Ensure they are flush with the side contacting the receiver. This is adequate for the .372 OD HKParts bushings, but 10mm original HK-build bushings should use a 10mm bushing and drill. It is acceptable to drill at .375 with the jig and ream to 10mm for this application.

PADDLE MAG AXLE PIN:
Currently set up for a 3/16" axle pin since those are the ones I use/make. Typically these are 1/8", but a 3/16 axle pin lets you cut up and ream out an original bushing and front push pin to use instead of buying the HKparts ones. Comply with all federal regulation.

MAG RELEASE PIN:
There are different sized bushings for L and R halves. Use the correct one. Press them in only deep enough to not run into the magwell geometry.

PARTS LIST
Optionally included (italics) are the odd-sized drills and reamers you might need. If you already have these or intend to use different sizes, they do not need to be purchased.

1    2    Each    2901A144    Black-Oxide High-Speed Steel Drill Bit Jobbers', B Bit Size, 4" Overall Length    Delivers tomorrow    5.64    11.28

2    2    Each    28255A53    Black-Oxide High-Speed Steel Short-Length Drill Bit 8 mm Bit Size, 79 mm Overall Length    Delivers tomorrow    6.52    13.04

3    1    Each    3671N149    Press-Fit Drill Bushing 0.315" ID, 1/2" OD, 1/2" Long    Delivers tomorrow    8.84    8.84

4    1    Each    96511A874    Press-Fit Drill Bushing 0.238" ID, 13/32" OD, 3/8" Long    Delivers tomorrow    6.84    6.84

5    1    Each    8803A601    High-Speed Steel Round-Shank Reamer with Straight Flute, 0.3130" Reamer Diameter    Delivers tomorrow    29.56    29.56

6    3    Each    6391K165    Oil-Embedded 841 Bronze Sleeve Bearing for 5/16" Shaft Diameter and 7/16" Housing ID, 3/4" Long    Delivers tomorrow    1.71    5.13

7    2    Each    8491A107    Press-Fit Drill Bushing 0.1875" ID, 5/16" OD, 1/2" Long    Delivers tomorrow    9.08    18.16

8    4    Each    8491A726    Press-Fit Drill Bushing 0.375" ID, 1/2" OD, 3/8" Long    Delivers tomorrow    7.68    30.72

9    1    Pack of 10 each    98381A591    Dowel Pin Alloy Steel, 5/16" Diameter, 2" Long    Delivers tomorrow    10.70    10.70