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Concept for upgrade study – Dboys M4 + RetroArms split gearbox with integrated Hop-Up chamber

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Concept for upgrade study – Dboys M4 + RetroArms split gearbox with integrated Hop-Up chamber

17. 05. 2017

We wanted always to show, how simple and easy is installation of our CNC gearboxes. So we bought airsoft replica by Dboys company – model M4A1 CQBR and we installed into it our CNC gearbox ver. II with integrated Hop-Up chamber. We did not want to change any inner parts, we wanted only to change gearbox, what was unreal in the end, because inner Dboys parts are low quality, but about that later.

Before disassemble, we did some chrono tests with 0,2g Excel BBs.

Next step was disassemble of DBoys gearbox and cleaning inner parts from weird lubrication.

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Assembling of lower gearbox part

I always start with bushings. The size of the slots is 7,98mm, little bit smaller because of bearings what have will to drop off, their precise size is 8,00 mm and if we do a bigger slot, they would drop off. I got a sample of CNC hardened bushings by American company F.L.T. Airsoft. I was quite surprised that these bushings were 7,96mm wide and they did not hold in the slots (we think that size of them was changed because of hardening process). So I used Loctite 2700 glue. I have a great experience with this glue and after drying it has great solid strength.


Next step was to shim the gears as it should be in the right way. We used original DBoys gears and we were surprised, because original gears are thinner, so we had to use a lot of shims. Than we removed gears and started to installation of selector plate, trigger safety, trigger switch with cut-off lever – all parts were original DBoys. Trigger switch with wiring fits perfectly. Only one need of customization was with cut-off lever, what had too big screw housing, so after tightening it won’t move. I had to reduce this housing and then it move without any problem (if you use our RetroArms cut-off lever, there would be no need of customization). I placed gears back into the gearbox, lubricated them with high quality lubrication and next thing was installing anti-reverse latch. There are little grooves from first quarter of 2017 for anti-reverse/trigger safety/trigger springs, so they don’t drop off and installation is much easier. Last thing was assembling DBoys trigger. Lower gearbox part was completely assembled and with typical CLICK noise I knew, that everything is alright.

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I installed lower gearbox part into the body. There was no need of grinding or any other form of customization. Pin holes were exactly where they supposed to be and they fits with gearbox without any problem. Assembled hand grip was without any noticeable gaps. Motor fits in hand grip without problems and can be assembled with hand grip cover. I also assembled rest of the body parts.


Assembling of upper gearbox part and Hop Up chamber

The upper gearbox part has integrated Hop Up chamber, so from original one I only used bucking and spacer. Original C-Clip lock was damaged from manufacturing (because material was cheap polystyrene). First problem was with bucking, because of higher collar, so when we assembled her, there was no space for C-Clip lock. So we used another no-name black bucking and she fits perfectly. We had to only shorten her for 2mm, so this bucking could be used without collet nut and C-Clip lock. We had to do this customization, because all of these elements would not work with our Hop Up chamber. Next step was assembling collet nut with inner barrel, inner barrel would not move anymore because of it. Also cylinder set was assembled too. There we saw how poor quality are these low-cost parts, mostly cylinder head. Cylinder head was polymer with metal nozzle tuber, these two parts are not from one piece but from two, so there was a huge air leak. I tried to glue them together, but when I used lubrication, it has problems with glue, so that did not work at all. So I changed this cylinder head with our RetroArms CNC cylinder head. Other parts like: nozzle, piston head, piston and cylinder were original DBoys. All of these parts were assembled into the upper gearbox part. One interesting thing, DBoys springs are shorter than usual ones. Our gearbox has quick spring change QSC system, so using original spring guider is not possible.

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One of advantages of split gearbox is that you can test fire you gun, without need of full completation. Inner barrel hold perfectly because of collet nut. Fire modes SAFE – SEMI – AUTO works without any problem. We test-fired a lot magazines without any problem.

Weapon Assembling

So all parts are ready to go, I assembled lower and upper parts together. Again, without any problems. Dust cover works (opening and closing), charging handle works without any problems.

Test shooting

The performance of the gun had leaks about 3m/s because of used original parts (only cylinder head was replaced). Nice surprise was increasing gun‘s performance up to about 7 - 10 m/s. Basically we did not do anything, so this increase is nice surprise. If we would continue with customizations, we believe that there is a lot of space for better performance. This is another advantage of split gearbox with integrated Hop-Up chamber. It can solve problems with straight body/gearbox/chamber line and increase the performance. (But big disparity of the body can also make this gearbox unreliable, but about that will be another article).

We did some chorno tests with 0,2g Excel BBs, gun was powered with 7.4V LiPo.

We wanted to show you, that our gearboxes can be assembled into the Chineese low-cost airsoft replicas.

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