DIY P90 Sling Adapter

The biggest milsim event in Finland is coming up again, and just like last year, I'll be part of the HQ staff. If there was a lesson learned from last year, it's that you need very little gear when you're sitting in a tent most of the time. As a result, I thought I'd take my latest project, the P90, with me.

Alas, the gun has no sling point to speak of. After a bit of research, I found some examples of how you can sling the gun. As it happens, I had previously saved some webbing and plastic buckles from a pair of trousers I had to discard. A thought occurred.

These bits would be turned into a sling adapter in what was to become my first tactical gear sewing project. Luckily, we have a sewing machine handy.

The first bit was simple, just sew the female buckle onto the webbing.

Next, I sandwiched the D-ring between the buckle and the two layers of webbing. I would rather have used a plastic D-ring, but this was all I had available (and I was eager to get going).

It took a few tries to get right.

The flip side is not the prettiest, but it works.

The top side looks quite nice for a first ever project.

The next step was to thread the webbing through the hole on the underside of the gun and tighten it so that when the buckle is closed, the webbing is tight.

The loose end could be pushed into the same hole.

It's not exactly Tactical Tailor quality, but it's mine. I'm pretty happy with this little project. With a single point sling, the gun hangs under my arm at exactly the spot where my hand reaches the grip instinctively.

Do you have any sewing projects you've done? Please share in the comments below.

Fixer-upper Part 4: Classic Army P90

Here we are again. A broken gun up for grabs for cheap on a For Sale site. And grabbed it I did.

Patient: Classic Army Sportline P90
Symptoms: "Has had some electrical issues, been to a tech but stopped working again quite some time ago and has been unused since"

I've never owned a P90 before, but it has been on my get-one-some-day list for some time. This one is from the Classic Army Sportline series, meaning the externals are mostly plastic. Now, the real P90 is mostly plastic as well, so it did not bother me that much, even though usually I'm a stickler for realism in materials. Compared to a real P90 the only thing not of similar materials is the upper receiver. The outer barrel and flash hider are metal, as are the optics rails. Furthermore, metal uppers should fit right in if I ever want to get one.

The gun came with a Swiss Arms C-more style red dot (with a missing front clamp), two hicap mags, two NiMh batteries and a dumb charger. The internals had been upgraded to some degree, but the previous owner did not know much about them. In his words, he didn't know what had been done, the gun had been to a tech twice for electrical problems and now just would not fire at all.

Troubleshooting

It didn't take long to find out that the batteries had simply died. With a fresh 7.4 V LiPo this thing came alive, if only on full auto. Something was wrong with the semi function, and I could hear a rattling sound coming from the inside when the gun was tilted back and forth.

Externally everything seemed good apart from a small crack in the rear of the receiver. The two sides of the front grip also did not quite stay put due to a screw having lost it's threads. Nothing to hinder functionality, that is.

The P90 turned out to be exceptionally easy to disassemble. As you might know, my first gun was an AUG. That was very easy to gut, but this one was even better.

After popping the back plate, you see two screws. Take those out, remove the plate they held down, and the gearbox comes out.

The P90 uses a less common V6 gearbox, so replacements and spare parts are somewhat harder to come by than on your average AK or M4.

The upper receiver can also be removed simply by pressing a rectangular button on top of the receiver and sliding the whole assembly out. I could not help but fall in love with the design straight away.

The modularity was not limited to the externals. The gearbox comes easily apart as well, separating the motor cage, wiring and trigger switches into an assembly away from the main body. No wires to stop you from working, and you can test everything without inserting the GB into the gun.

The problem with the semi auto not working quickly became apparent once I peeked into the trigger switch assembly. A part had broken off from the trigger trolley (seen above still wedged below the trigger contacts). With the piece off, the trolley would simply skip over the trigger switch, not move at all and thus not engage the contacts.

The rattling I heard when tilting the gun was not an extremely short spring as I though at first (the gun chronoed just above 0.6 J), but rather the broken stem of the plastic spring guide sliding inside the spring.

The two main problems had been identified, and I got to work fixing them.

Fixing-upping

In addition to fixing what was broken, my plan also included swapping the plastic bushing for steel ones and re-shimming, fixing AOE, installing a MOSFET, and putting in a flat blade fuse and deans connectors. My plan was to build a CQB beast with a high ROF and moderate power.

Swapping out the broken spring guide seemed rather straightforward, as I quickly found a steel spring guide in my spares drawer. The V6 box takes V2 spring guides, which makes it easy to replace. Or so I thought.

A couple of test shots later the spring was compressed into the piston, which in turn was stuck in the rearward position. In case you've never done it, let me tell you that disassembling the gearbox again with this kind of a mine inside is just lovely. I managed to get it out without issue, luckily, only to have it get stuck again (because of course I had to try it again). It was pretty clear that the spring guide would not work here.

I scavenged another spring guide from an M4 parts box I had lying around, waiting to get assembled. It was otherwise similar, but the end was tapered and, more importantly, it had a diameter that was 0.21 millimetres smaller. The one fifth of a millimetre was all that was needed, the spring did not get stuck. The spring, as well as I, was free to move forward.

Swapping out the plastic bushings for steel ones was pretty straightforward, but they did sit quite low in the shell, and I had to use more shims than I've ever had to before. At some point I began to question whether or not I was doing something horrendously wrong, as the the combined thickness of the pile of shims on the spur gear crossed the 1 mm mark. It worked swimmingly, though, so that's tolerances for you.

Trying to fix the angle of engagement on the piston turned out to come with its own set of problems. The piston head was attached with a tiny nut sitting free inside the piston. I could unscrew the head with ease, but lacking a socket wrench for ants I could not put it back on. Who makes these things? I opted instead for an aluminium double sealed head I had lying around.

My problems with the spring guide had apparently resulted in some structural problems, since at some point during test firing the steel tooth broke off the piston. Luckily I had an old spare nylon piston with it's own plastic piston head that went in after a quick AOE fix, similar to the one pictured above.

My first attempt at fixing the trigger trolley consisted of glue and a several thin layers from a piece of paper towel. I figured a glass fibre style layer cake of material and bonding agent would result in a strong base which I could then file into shape. 

Too bad the plastic wasn't very glue friendly. The glue soaked paper held together fine, it just would not stick to the plastic hard enough to withstand the strikes delivered by the cut off lever. The same exact thing happened with various attempts made with super glue, epoxy and even chemical metal. This was one piece of engineering I was not exactly happy with. A thin piece of plastic subjected to repeated strike forces sounds exactly like a recipe for success.

Since the original plastic was a no-go, it was time to fabricate a stronger part. I chose a piece of sheet metal for the job with roughly the same thickness as the original plastic. After tracing out the shape of the trigger trolley, I cut it out, filed it down to size and rounded off any rough edges.

I drilled a few holes (with a 1.5mm drill bit) into the metal, filed down the original trigger trolley, drilled some holes into that as well and roughened up both surfaces, hoping the epoxy would finally bond well enough. I chose to make the holes canted every which way so that once the glue set, the pieces would not come off each other so easily.

To further address the issue, I cut down the trigger contacts so that the trolley would have more freedom to move up and down (the top piece is cut to shape above, the lower shows the original shape).

Everything was working pretty nicely apart from the fact I was getting triple and quadruple shot bursts on semi. The culprit in the end turned out to be my upgraded trigger trolley. The sheet metal was not as thick as the plastic, and thus the trolley was able to sit higher in its rails before disengaging. This in turn made it so that the cut off would not lift it out of place until the trigger was fully pressed down. I fixed this by epoxying another piece of the sheet metal onto the cut off and filed it to size. No more problems with semi auto! 

To round things off, I made a basic DIY MOSFET out of an IRLB3034 and some resistors. The middle prong is cut off and the negative lead is attached with a small bolt and nut to the back plate. Sturdier attachment, better flow of current.

Wiring up a MOSFET on a P90 can be done exactly like in the AUG, since the trigger assembly is the same. There's one switch and a pair of contacts for the semi, and another for full auto. It's only the movement of the trigger that decides whether the gun fires semi or full auto. On the AUG it's up to the shooter, on the P90 the fire selector limits the movement of the trigger.

All the extra bits and pieces fit in without issue and the gun was mechanically ready for service.

One final fix was to repair the cracked receiver. Since the material is ABS, you can forego glue and use pure acetone, as it melts ABS. My end result is a bit sloppy, but with an unhurried approach, slowly wicking the acetone into the crack, you can achieve a brilliant result. While mine isn't aesthetically as pleasing as it could, the upside is still that the plastic is again one single piece as it was when moulded, not two parts held together by an adhesive layer.

With an 11.1 V LiPo this little beast is pumping out 33 rounds per second at 0.92 J. Right on the money, if you ask me. Now I only need magazines that aren't hicaps and can still keep up with that.

Questions? Comments? Anything you'd add or like to know more of? I'd love to hear from you! 

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.