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SRC M4 (custom AR-15)

#1 User is offline   aotsukisho 

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Posted 05 June 2020 - 05:29 PM

Picked up a cheap gearbox on auction, turns out it's an APS quick spring change model. I've heard of it be referred to as the APS Hybrid gearbox, as it's compatible with their electric blowback models.. APS is not known for their internal component quality so we shall see what we can do to help this gearbox survive.

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Here is the gearbox as received. One of the bushings fell out but all six are there, along with the quick change type spring guide.

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First things first, loctite on the spring guide. Looking at photos the new gearbox is supposed to come with a bearing or bushing or something for the spring to rest on - this one did not. However, I don't think this will be a problem. Due to this gearbox's fragility I will be using an M110 spring and full cylinder with it, and limiting the spring preload might be a small factor but every little bit helps with keeping this thing alive.

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Stock 8mm brass bushings, these will be changed out to 8mm steel SHS ones for longevity and stability

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The original seller actually cleaned this out quite well before sending it to me, bravo and great job.

Also note the useless 'reinforcement' that APS has done near the gears, all this will do is limit compatibility and add weight.

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Popped out the stock bushings, quick external shell shot

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Stress relief work done to the front

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Don't mind the chatter marks, my dremel bit slipped off a couple of times lol

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Gluing in the SHS bushings. Hopefully superglue will hold this low-stress build, but if they start moving again I will probably buy the cheaper sintered bushings and JB Weld them in. If this gearbox blows up, I won't miss $3

Fitting this into my budget SRC tan receiver set this seems to fit very well compared to other gearboxes I've tried. I think I will build this gearbox and try it out in that one to see how it performs, using a long barrel and full cylinder to extract as much power as possible out of the M110 to keep the stress on the gearbox minimal. I will be reusing as many stock parts as possible to help keep my surplus of replaced-parts minimal seeing as how any new build 90% of the time needs me to buy new stuff.

Right off the bat, the Classic Army gears that I'm using to properly align the bushings while being glued don't fit, the spur gear is too tall and has a ton of resistance spinning when the shells are screwed together. I will have to open up my stripped SRC gearbox and have to reuse those internals perhaps, or remove teeth from my SHS SR-25 set assuming those fit. Time to experiment.
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