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Last night, during an airsoft indoor CQB, my beloved APS ASR106 Mini Patriot M4 pistol rifle failed. Its trigger stuck in the safe position even though I rotated the mode switch to semi-auto and full-auto. After getting home, I took apart the gun and found the failure to be inside the version 2 gearbox. Disassembling the gearbox, I found that the trigger post had broken off.
Examining the trigger mechanism, I realized that the trigger post is molded as part of the gearbox. It prevents the trigger contact switch from traveling too far back. In semi-auto mode, when the sector gear moves to a certain position, it trips the semi-auto level arm, which lifts the contact switch off the trigger and snaps back to the trigger post. Repeated hits on trigger post in this manner causes the pot metal to break. And the gearbox is not an inexpensive part to replace.
Why they don't make this stop on the trigger mechanism itself is beyond me. After learning about this problem, I start to think up solutions for this common problem. In this article, I will detail out each solutions so that you can find the proper solution for you gun. I don't know whether other AEG gearbox versions have this same problem. Maybe you can chime in and let us know.
Replacing the entire gearbox or the gearbox shell is probably the most obvious and the most straight forward solution. However, it is definitely not the most inexpensive solution. At the time of this writing, APS gearbox shell costs $40, while the entire replacement gearbox costs $78.
Replacing the entire gearbox is the most expensive method, but it is the easiest, because you don't have to take the gearbox apart at all. However, if you go the gearbox shell method, then you will have to do a bit of work transferring parts.
Replacing the gearbox is certainly cheaper than buying a new airsoft gun. But if you are looking for even cheaper solutions, then read the next few sections below.
I came up with the ideal of using a wedging a safety pin on the trigger contact spring post and in between the contact switch and the trigger. This was a relatively simple and reliable solution. It required you to have a small safety pin (see photo below) and a strong wire cutter.
Cut out a portion of the safety pin to include the circular loop and a leg just long enough to go from the spring post to the trigger. The photo below shows the wire cutter and the three parts after the cut.
Place the safety pin loop onto the spring post, then wedge the leg in between the trigger and contact switch. The photo below shows the correct installation. Once you complete the installation, you should have fixed the trigger post problem.
The only problem is that the gearbox now only provides two modes: safe and full-auto. The gun is in full-auto whether you put the selector switch to semi-auto or full-auto. That is because the safety pin no longer prevents the contact switch from snapping backward. This is actually a very reliable side-effect. To get semi-auto back or improve this solution with burst control, see next sub-section.
One thing we have always wanted for our airsoft gun is burst control. So after fixing the trigger post with the full-auto side-effect, I have decided to add a burst control unit to the gun.
I looked at a number of different burst control mosfet units and picked Burst Wizard King Kong Elite. There was a number of factors that helped me choose this route. One, most other burst control units costs ~$100, Burst Wizard King Kong Elite costs $40 as I write this article. Other units in this prices range are not available for purchase. Burst Wizard King Kong can be plug-and-play inline with the battery with permanent soldering as an option.
For in-depth review of the Burst Wizard King Kong Elite, see our "Burst Wizard King Kong Elite Review" article.
With the Burst Wizard King Kong Elite, we were able to set the number of burst to emulate semi-automatic. We can also set it to three shot burst to emulate the M4/M16 used by the U.S. military. With Burst Wizard King Kong Elite, you can also hold down the trigger to turn it into full auto.
The only trouble we had with Burst Wizard King Kong Elite performing semi-automatic was the sensitive setting. By default, the Burst Wizard King Kong Elite will cycle once and pull the piston back halfway. That's halfway too far from the piston resting position. The correct sensitivity value takes time based on trial-and error. Setting it to the value 9 seems to do the trick for us. It is still not exact, but much better than default.
After explaining the trigger post failure to my friend, he suggested drilling and tapping the gearbox to use a screw as the trigger post. That was a great idea. This idea required a little more tools, like the drill and the tap. In addition, it required you to find the right screw for the job.
Afterward, I read several Internet postings of folks drilling through the gearbox to use a metal dowel. It was a good idea as well. This idea still needed a drill, but not the tap. And it still required you to find the right metal dowel.
The possible downside to this solution is that the contact switch will continue to strike the screw/dowel. Will anything--contact switch, dowel, or gearbox--eventually give is a question.