Tuesday, 1 March 2016

Machinegunning - Phase Two

In the previous post I introduced the project, and have now finally had time to actually get back on the horse, so to speak.

The previous post ended with a to-do list as follows:

  • Replace internals (I already have a new spring and some leftover torque gears [21.6:1] as well as a full cylinder, a new piston and head, o-ring nozzle and steel bushings)
  • Wire to the rear, install MOSFET and deans (I have a G36 collapsing stock that will house the large battery)
  • Fix the hop-up (the current one is pretty dicey, I'm planning on installing R-hop for range)
  • Pretty up the externals (I intend to get rid of the engravings somehow, tips welcome)
  • Acquire battery (3600mAh 7.4V LiPo on order) 
  • Acquire bipod (standard G36 bipod on order)
  • Acquire magazine (an electronically run G36 C-mag would be ideal, tips on brand welcome)
  • Rewire magazine so it feeds when the gun fires and runs off the gun's own battery

Since last time, I've managed to acquire all the requisite parts and got well under way on the building. Let's break down the list above step by step.

Internals replacement

I was going to go with what I thought were Modify 21.6:1 torque gears from a boneyard gun, but it later turned out they were actually standard 18:1. I had based my assumption of the gear ratio on a calculation derived from the RPS of the abovementioned gun, but it later turned out the RPS was a result of poor electric conductivity in the wiring harness, i.e. the gun ran slower than it should have. A curious aside, but let's move on. 

I had in store a set of 18:1 steel XYT gears that I went with in the end. The spring I installed was an SHS M150, which I hoped would be putting out around 2J of power with a sorbothane pad installed. I went mostly with SHS for other parts as well. The cylinder, piston, steel spring guide, selector plate, steel bushings and motor were all of SHS make.  

I followed the to-do list above otherwise, but found out the metal nozzle with an o-ring I had for a G36 did not fit this particular gun.

The nozzle in the front position was woefully short, so I went back to the stock nozzle the gearbox came with. As it later turned out, that did not hinder performance much, if at all.

The selector plate was slightly too thick to fit the rails, so it had to be filed down slightly.

I also thinned it down on the parts that touch the rails to make it run smoothly. A bit of silicone grease finished the job and made the selector switch turn extremely smoothly.

Otherwise the building of the box was rather straightforward, but I'd be lying if I said closing it with the M150 spring was anything but challenging. I think I got everything to stay in place on the fourth or fifth try.

And then the motor would not turn the box all the way over. After considerable time and trouble I finally figured out the problem had to do with there simply not being enough physical space in the box for the spring to wind fully, as I had used both a spring guide and piston head which had ball bearings. I opted to pull out the ball bearings from the piston head and replace them with a simple metal washer. Once I got the box back together again, this time I got it to turn over, finally. It was time to pull out the old wires and rewire the whole thing.

Rewiring to the rear

To begin with, I desoldered the old stock wires that had run to the front handguard. One very positive thing about the V3 gearbox is that the wires attach to the outside of the box, so I could do all this work without opening the gearbox back up again. The plan was to build a completely new wiring harness and install a MOSFET, a fuse and a deans connector along the way.

The original wiring was pretty shot up to begin with, and I had specifically purchased silvered 16 AWG wires that had a PTFE jacket for this project. The overall thickness of said wire is significantly less than same gauge wire with a silicone jacket. It is also stiff, so it keeps its shape when bent, which is great for threading it through small holes and fitting the gearbox in the gun's frame.

I did some measuring to see how much wire I'd need to complete the harness and have it run all the way to the rear of the stock.

The measurements didn't need to be completely exact, but with wires too short I'd run into trouble trying to hook up the battery later on.

There are plenty of guides on building DIY MOSFET units, one of which (in Finnish) I followed. Rather straightforward. The unit itself was an IRF3703.

Sadly the unit (a cheap Chinese knock-off, as I later found out) did not last long in use and in the end I replaced it with a much better IRLB3034. The "fun" thing about MOSFETs is that they typically fail  (burn out) so that the circuit remains closed, which means the gun will fire full auto, regardless of trigger pull or selector position, as long as the battery remains hooked up. That's not really something you want to happen mid-game, so I suggest forgoing the cheapest pieces on ebay and getting your components from a proper electronics store.

This build was fantastic in the way parts almost magically fit together. I used a 30A blade fuse in the build, and completely by accident found out not only that the holder was exactly the correct size to fit through the opening in the rear of the stock, but also that it would fit partway out of the side of the stock where it contacts the receiver when folded, making the removal of a burnt fuse very easy.

The battery also fit in the compartment with the MOSFET, fuse and wiring like a glove.

With the wiring and internals otherwise done, it was time to move onto the hop-up.

Hop-up bucking replacement and R-hopping

There are plenty of guides on how to build an R-hop, so I won't link any particular one here. For this project, I opted to use silicone tubing of my choosing and cut it into shape instead of buying ready made patches.

To build an R-hop, you basically cut a patch that will fit in the barrel window and glue it in place. Next you take your bucking, flip it around and cut and sand down the nub so that the inside surface is flush.

I used a green G&G bucking which I pulled over a spring guide in order to have a nice, hard working surface. Once flat, the bucking was once again flipped the right way round and placed on the barrel. I used some silicone grease on the outside of the bucking in order to get it to slide effortlessly into the hop-up mechanism.


I went and purchased an original JG bipod meant for the G36 to use in this project. Sadly, it came without any attachment hardware, so I had a gun and a bipod but no way to connect the two.

After rummaging through my hardware drawers for a while, though, I came across an interesting nut and bolt that were leftovers from an IKEA wardrobe installation, meant for holding two separate cabinets together side by side.

As if by miraculous coincidence, these two fit the hole meant for the bipod not only in thickness, but also in length just perfectly.

With the bipod in place, the gun was as complete as I was going to be able to make it at this point. I topped it up with a non-magnifying red and green dot ACOG I had set aside just for this project.

Testing and wrapping up

I now had a working gun that I could finally test. The G36 has a nice, large receiver, so all the wiring and so on fit in without too much effort. In fact, it is one of the easiest AEGs to take apart and put back together that I've come across so far.

I set up my chrono for a test firing session so I could see how the gun would fare. 

My expectations (and local machine gun power limits of 2J) were exceeded rather wildly. The SHS M150 spring was giving out way too much power, and has to be substituted for another if I intend to actually field the gun at some point. For now, though, I'll keep the spring in and test how the R-hop works. It might also be that the spring loosens up over time, so I don't want to rush in to swap it just yet.

I already also have an electronically run C-mag that fits the gun, but it still requires some extra work. In the end, though, I will have wires running from the gun's motor to the mag so it will wind whenever the trigger is pulled, thus obviating the need to push a separate button or to have batteries installed in the magazine itself. All in all, though, most of the work is already done and the gun is already usable!

The externals still also need a bit of work, as the ABS plastic has worn down some and the surface has ugly engravings on it. At this point I'm planning on using a sponge with acetone to recreate the uneven surface of the original parts. Any tips on that are more than welcome. 

For now, though, stay tuned, and if you feel like it, do drop me a line regarding anything relating to this build or anything else on the blog!

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