The Ugliest Hack I’ve Ever Done

by Zach Billings on March 31, 2011

KWA GearboxVery few of you are likely to have any real understanding of the inner workings of an electric airsoft rifle. Those with a little knowledge might know that you have a set of gears, driven by a motor. Those gears move a piston which, when released, uses spring tension to create a burst of compressed air. It’s that burst of compressed air that fires the bb from the gun. Only a handful of people with a lot more experience can typically look at the rest of a gearbox’s contents without having to scratch their head.

Recently, my brother’s gun has been failing to fire at times. At this point, I have enough detailed knowledge regarding KWA AR-15 variants that I can diagnose problems without having to disassemble the gun. I knew the problem was being caused by a wiring issue. It wasn’t a short, as the motor wouldn’t even try to spin when the trigger was pulled. It was either a loose connection or pinched wires. Upon removing the gearbox from the gun, I found nothing but a couple moderately pinched wires to indicate a problem. What complicates the matter is that I once installed a MOSFET in my brother’s M16. The MOSFET is meant to generally improve electronic functionality of the gun, by reducing wear on electrical components, adding programability, and more. The MOSFET was adding a variable that I can’t diagnose. It was time to rewire the gun and remove the MOSFET from the equation all together.

V2 GearboxTo this point, wiring was the one area of the gun that I have almost never had to work with, and while I’m familiar with how it works, I’m not comfortable with it. Upon opening the gearbox, I discovered a hairline fracture in the gearbox casing. This fracture is common on older KWA models and will eventually expand to a compromising crack. Not a good sign, but not an immediate concern. I move on to remove the gearbox components so I could get to the trigger assembly, wires, and electrical contacts. This bit is an easy task that requires little attention and only a few minutes of work.

My plan was now to remove the trigger assembly and wiring harness all together. I would then swap in my old M4 wiring harness and rewire the gearbox to the rear for an M16 configuration. Since I’d never had to rewire from M4 to M16 configuration, or vise-versa, I had to open up a friend’s M16 to double check the wiring setup. I was pretty sure I knew where everything needed to go, but a quick check confirmed my beliefs. A few solders and a little finagling later, and the rewire was done.

I assembled the gearbox and put it in the gun. I went to test fire it and it got the first shot off, but then suffered what I assessed to be a severe mechanical jam. A mechanical jam is the worst thing that can happen to a gearbox. It means something solid has broken in the gearbox and has become lodged in the teeth of the gears or piston. If you have very strong gearbox components (i.e. gears, piston, etc) this hopefully only means one part is broken. Usually this wouldn’t be a concern, since I have most of the parts for a KWA gearbox in storage.

When I opened the gearbox, I released the spring tension and then found nothing wrong. The trigger contact unit was sitting in a strange position, but it reset without a problem. After a second check, I found nothing wrong and reassembled the gearbox. Just as I went to put in the screws to tighten the gearbox, I noticed a little rectangular piece of metal sitting on my desk. When I looked closer I could see the distinctive look of sheared metal on one side. The broken part. I opened the gearbox again and located where it came from. It turned out to be a post that holds the trigger contact unit in place. This is the infamous trigger post that KWA owners with older gearboxes have been fighting for years. They have a tendency to break. It just hadn’t happened to me yet.

Version 2 GearboxI was stumped. We had a game coming up in four days and there wasn’t the time or money to get a new gearbox shell in that time. The gun won’t shoot without the trigger post and there’s no way to jury rig it without compromising structural integrity. After about 10 minutes in thought, I opted to attempt the ugliest hack I’ve ever done (to quote Link from the Matrix). I ran down to the basement and dug up one of my old KWA M4 gearbox shells. Gearbox shells are comprised of two halves which split apart lengthwise. The left half (in relation to the way the sit in the gun) is the side that all the components seat into. The right half is then seated onto everything, to hold things in place and shield the components. The left half was the broken one. My two old shells (each a complete shell with both sides) were cracked catastrophically where the hairline crack on the M16 gearbox was developing. Together, the shells were completely useless, but I decided to try using a left half with a crack but healthy trigger post. I would then use the healthy right half to help hold the major crack on the left half closed.

After a total of four hours of work. It was finally together and shooting. It’s ugly, and definitely a band-aid, not a fix, but I think it will work for at least one game. People may call me a douchebag when I boast and brag about my ‘airsmithing’ skills, but no one can argue with results like that.

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