Saturday, 5 December 2015

Magpul Mishmash

My latest build is another M4. As much as I've grown to loathe the v2 gearbox, I'm pretty happy with how this one turned out in the end.

The whole thing started out as an externals build. Ever since I got the G&G M4, I had been on the market for a metal M4 receiver. The plastic one simply didn't do it for me. Before I got it, though, I had been buying second hand externals over time to spice up the appearance of the G&G. Once I finally happened upon a fairly cheap used upper and lower receiver set that also came with a gearbox and motor, I had enough parts to build a whole new gun. So I stripped the G&G and began work on the new gun.

The Magpul handguard (though actually an ACM copy, I believe) was very nice to hold, but came with one or two problems. Namely that one, it was too thick to fit directly inside the delta ring of my current upper receiver and two, it was longer than where the front sight sat on the barrell I was using.

The first issue was rather quickly solved with a bit of filing. I used some vinyl tape to protect the parts I didn't want to scratch.

The second issue took a bit more work. The original front sight had to go.

I didn't want to get a whole new front sight, so I simply lopped of the top part with a saw and began to file down the rough edges to make it fit inside the handguard.

As usual, I got a bit carried away with the shiny metal surface and in the end I filed the whole gas block down to a shiny, smooth piece. I also removed the sling mount and bayonet lug as they would not be needed inside the handguard.

And in the end, like a carefully crafted scale model plane cockpit, it's hardly visible once done, only bringing joy to the original builder.

One further problem reared its head, and it was the fact that the handguard would not actually now stay in one piece as the retaining ring did not have the gas block in front of it.

I settled this in a simple manner and screwed on a nice looking suppressor that pressed the ring down and locked the handguard in place. This might not be the correct way to do it, but if it works, what the hell.

I also wanted to replace the trigger guard and pistol grip. My hand is not designed to work with the original AR series pistol grip. The finger shelf simply seems to sit in a completely wrong place. I opted for a nice looking Dytac HK416 style pistol grip. Obviously it would not fit directly onto the lower, so I had to file the upper edges of it down by approximately one millimetre all around, and also slim down the lip protruding onto the underside of the trigger guard.

The rotary tool slipped a couple of times, but always in the direction that does not show, so all was well.

The trigger guard I got was an FMA one in olive drab (it was the only colour on sale). Luckily the material was slightly porous, so a couple of layers of spray paint did the job splendidly.

Once dry, the trigger guard fit into place without any modifications, which seemed almost unusual at this point.

The last bit to add was a Magpul PTS (authentic this time) CTR stock. The stock locking spring on it is from another world when compared to cheap airsoft stocks and it took some effort to push down far enough for the stock to slide onto the buffer tube, but otherwise it was such an undramatic event I didn't even take pictures.

One final piece to fix was the missing cover on the right hand side of the lower receiver. I bought some Dytac cover plates only to discover they're made to a wildly different specification than my lower used.

As is obvious, the inner diameter of the hole is much bigger than the raised portion on the cover plate. I opted to take the quick and mostly painless road of cyanoacrylate.

At least the outer diameter and height of the cover was similar to the detent in the receiver, so once the glue dried, everything was breezy. Super glue on metal is easy enough to break so if I ever need to adjust the fire selector, the cover is still removable.

Once I topped everything off with a Swiss Arms C-More sight lookalike, the gun was in fielding condition. The sight is nice in that is has built in iron sights, which this gun would otherwise lack. Not that they're very important in airsoft, but it's that extra bit of realism that really works for me.

I played with the gun in a few weekend skirmishes, and while very nice to handle and fairly accurate, there was something missing. At times BBs would fly off course for no apparent reason, and I was hoping of a bit higher rate of fire.

So, back to the workshop. I know calling a bit of table and a jumble of tools a workshop is overkill, but it makes me feel warm and fluffy and cozy inside.

The first thing to go was the nozzle that I swapped for an o-ringed one for better airseal. The ferrite motor that came with the gun went out and an Aim Top high torque motor went in. I had previously used the same motor on an otherwise stock MP5 and got a nice 20 rps out of it on a 7.4V LiPo. I was expecting the same here, since I was using a similar battery and identical 18:1 gears.

In order to fix the angle of engagement for higher rps I used a piece of 70D Sorbothane. I know 20 rps is not that high, but better safe than sorry. I'd also like to have the gearbox last as long as possible. The box had strengthening cuts made by the previous owner, so that was already taken care of.

The sorbo installation was pretty standard. Remove the stock pad, cut a piece of sorbo, glue into place and cover with the stock pad.

Yes I cut this beautiful piece myself, thank you. It fit into place as it should so I don't mind, altough a proper sized punch might be in order one of these days.

One obvious thing to go along with the motor swap was to reshim the whole gearbox. I had a hard time believing how high I had to set the bevel gear for the pinion to properly mesh.

The picture doesn't properly show it, but the bevel is sitting almost a full millimetre off the bushing surface. The rest of the gears were still surprisingly easy to shim so they meshed nicely, although I was afraid I'd run out of space somewhere, get poor contact on the piston or jam the tappet plate between the sector surface and the gearbox shell. That didn't happen, luckily, but I did have to work on the tappet a bit.

After I had shimmed the gearbox and was turning it by hand with all parts except the spring and spring guide inside, I noticed that there was a clear spot where the gears would not turn without significant extra force. I found out the tappet plate was hanging on the sector gear nub at the end of its cycle. I cut and filed it down a bit to make the action smooth again. Definitely a problem I've not run into before.

Before I got to testing the gun though, I found myself doing a bit more grinding and fitting.

The first issue was with the cutoff lever, which had a bit of wobble to it. The axle on which it sat seemed a bit high for the screw to properly hold it down.

After grinding down the axle about a millimetre, the screw held the lever in place a lot better without hindering the lever's motion.

A second, quite a lot larger area I had to grind down was at the back of the gearbox. I have no knowledge of either the gearbox or the receiver manufacturer, but they simply would not mix properly.

With the back of the box ground down, it would finally sit straight inside the receiver instead of tilting forwards. Before the fix, with all the screws tightened properly, the gun would not accept a magazine because the gearbox was tilting the hop up unit so far out of place.

Since I was on a roll and the gun had exhibited some inconsistency issues with the hop-up, I went all out and built my first ever R-hop out of silicone tubing which, admittedly, I had previously purchased for just this purpose.

The R-hop patch seemed to sit in the window rather nicely, but I have not yet had the time to test it properly for long range.

A final touch was to add a fuse and fuse holder as well as a deans battery connector next to the DIY MOSFET built by the previous owner.

With all the parts now done, it was finally time to wrap it all up for final testing. The gun would chrono around 1,4J, which was below what I was expecting. In the end the reason for this was that I had used the original ported cylinder, and the sorbo pad had diminished the cylinder volume enough for it to lower the output. Swapping in a full cylinder fixed this issue quickly.

The bigger problem was that I was only getting around 17 rps from the setup, which should have been putting out a steady 20. I disassembled the gun twice to get to the root of the matter. Was it the shimming? Was the motor angle somehow off? Was the battery dying?

In the end I discovered the problem to be the wire connectors used for the front wiring harness. They heated up like crazy when shooting more than single shots every now and then, so obviously they were hindering the flow of current quite a bit.

What I opted to do was to cut the old connectors off and attach new, more heavy duty ones on the other side of the hole beneath the barrel. What this meant was that the front wiring harness was now a fixed part of the barrel since the larger connectors would not fit through, but since I wasn't planning on using a second upper anytime soon, it didn't really matter.

The new connectors still allow for the upper to be removed and now there's no need to thread the wires through the hole every single time.

The larger connectors have just enough space to sit nicely next to the hop up unit and not interfere anything.

With the larger connectors, the rate of fire finally climbed to the expected 20 rps, and the wires would not heat up anymore. Even the trigger response felt crisper since the motor was getting all the current it required.

In short: You CAN get a cool looking, well working M4 on the cheap (all parts and materials total less than 100 EUR on this build), but you might need to mess around with it before its all it's cut out to be.

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