My first tested design of m4 mag coupler.  This product is intended to be used in the field, the design allows it to be struck and crushed without breaking easily, so you can play or train without worrying about not hitting your chargers. 

This design also allows correcting the mag feeding problems, especially with those mags that wiggle a lot inside the gun. In my case it happened with a real cap mag (30 round) with a very strong spring that moves the magazine a little back and causing a failure in the feeding.

Also, to prevent the coupler from slipping, it can be tightened with a bolt and nut, keeping its designated position.

It must be said, they are not the most beautiful, but they are functional!

To print i recommend:

I use a nozzle size 0.5mm, You can use 0.4mm, it depends on you.
PLA filament, due to its elasticity.
Maximum layer height of 0.25mm and minimum of 0.15mm, a height outside these ranges weakens the part.
wall thickness of 0.5mm, this can be modified but cannot be less than 0.4mm.
Infill Between 20% and 60%.
Top and bottom thickness 1.2mm.
Maximum print speed 80mm/s, I recommend a slower speed.
Nozzle temp 180°c.
Bed temp 60°c.
Need supports, i recommend use RAFT for good adhesion.
All of the above has been what gave me the best results, you can play with the measurements until you find the one that best suits your printer or PLA. My printer is a Creality Ender 5. Use a 1.75mm Cycla 3d Filament. 

You also need:
Metric bolt M4 x 25 or M4 x 20
Metric nut M4

AIRSOFT M4, AR mag coupler

M4, AR mag coupler

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AIRSOFT M4, AR mag coupler