New PTW Upper

Well… sort of. Two “new” uppers – the lower 14.5″ was just a simple swap of barrel nut, rail, and gas block from my other guns. The MK18 Mod 1 style upper (pictured above) is the one that I’d like to show off today (post field test in the snow, hence some rust that wasn’t quite cleaned off entirely yet).


I intended to build this upper with a couple of goals in mind – one of which was weight savings (hence the shorter barrel). The easiest way to shave significant weight from a PTW upper is to get a lighter, aluminum Dytac outer barrel. I’ve got a couple of notes on this barrel that I’ve either learned from The Reptile House blog or through observation of the fit with the other parts in this build. As much as some of you (“Jetlag”) may make fun of me for trying to make a lighter gun, I’m kinda lazy when it comes to slinging around a gun that’s heavier than it needs to be.

As I had a pretty specific look and feel in mind, it did result in some different parts choices – including Prime upper receiver, Madbull MK18 rail, Dytac 10.5 barrel. Internal parts were chosen mainly for testing purposes (hopefully for the better) – FCC Advance Gen 2 hop chamber, MAG Curve Roller (already tested), Orga 6.13 barrel. I’ll discuss these parts further, after the break.

Start with one Prime upper receiver. As it turns out, I’ve got two different generations of Prime uppers – one of which is darker in colour, has fractional side to side play in my Systema lower, and was originally purchased a few years ago. The bottommost one was recently purchased from EhobbyAsia and features a more grey tone and a better fit in my lower. Both feature the same machining from what I can tell, both came in Prime boxes with the same sticker on them, both dummy forward assists (included) appear identical.



First step in starting this build: acquire Madbull Daniel Defense RIS II MK18 Rail (FDE), which proved to be a more difficult task than I had imagined (timing was just right on this, as had just restocked these as I was looking around for one). As the lack of mock gas tube is more realistic, yet entirely useless on an airsoft rifle, I still did want to incorporate this onto my build for that extra bit of looking realistic – I mean, I spent all this money on a “Training Weapon” as is, might as well go the extra few bucks to integrate this dummy part. Installation of a dummy gas tube through most Madbull rails requires modification to the rail system – in this case, it’s as simple as drilling a hole out. Nothing too complex here; I’ve done this a couple of times before.

Not pictured here is a Madbull Daniel Defense Low Profile Gas Block that I’ve had for ages, and scrounged from my last MK18 Mod 1 build that utilized a rail on loan (“Dizzy” requested his rail returned to him years ago and I ended up disassembling and selling that AEG, save for a few small parts).


As it turns out, the Madbull Daniel Defense MK18 Rail barrel nut is threaded for Tokyo Marui AEG’s – the thread pitch is off, contrary to some posts all over the internet. This means that this barrel nut will not go onto a Systema PTW without chasing the threads (read: really just forcing it on) – believe me, I tried. The search for a barrel nut that would fit Prime’s threading (the same as that found on a Systema PTW) was on… considering as how Madbull’s barrel nut for G&P receivers is next to impossible to find anywhere, and how Daniel Defense just simply wasn’t returning my inquiries about ordering just a barrel nut, desperate measures were in order. I did manage to secure an aftermarket one direct from Japan – thankfully it turns out that I know someone living there at the moment who was willing to put through an order for me to a store I’ve visited before, as well as I know someone else who was more than happy to help me translate an email message to that shop to ask which model of barrel nut would fit Prime (read more about this here).


Next two steps were to make the rail fit and function properly. Pictured above is the modification done to the lower corner of the rail system; this allows the receivers to be shotgunned open – thanks to pointers from “Mouse” and “Jester” for explaining how they did it to theirs. Secondly (below), the top corner of the rail system needed to be filed down, where it would ordinarily sit near-to-flush with the upper receiver to form that continuous upper rail. This would allow the rail to sit straight on the upper receiver, as otherwise the rail would cant slightly downwards as a result of this corner in question being butted up against the upper receiver.


Acquiring a Dytac 10.5″ barrel through a trade was quite easy, though I didn’t end up getting the Mil-Spec profile I was hoping for. As per The Reptile House’s repost of information straight from Tackleberry about modifying Dytac outer barrels to run the Systema pin: the grub screw was removed, hole drilled out, and proper Systema Knock Pin glued into place. Reptile House’s post has a better explanation of this, but essentially this keeps the barrel (and thus hop up unit) from rotating, which could potentially cause feeding issues.


From here, I had actually ordered an Orga Magnus 6.13 barrel with that Japan parts order previously mentioned – and was a little disappointed to find that the Orga inner barrel doesn’t actually fit snug enough in the Dytac outer barrel. Turns out that this isn’t just my gun – Reptile House has reported similar findings somewhere on his blog. Though it may not actually matter if the inner barrel floats around in there a bit, desire to “fix” the inner barrel not sitting in exactly the same spot every time a BB flies down it kicked in and I just had to do something about it… teflon tape, wrapped around the inner barrel. Works on AEG’s, don’t see why it wouldn’t work for PTW (I’m not sure if this actually makes a difference or not, to be honest).


Considering how this new upper was also being put together to go with more current US Mil. impressions in mind, the whole Madbull Daniel Defense MK18 rail fit just right. I have been noticing that forward grips are in pictures of 2012+ MARSOC operators less and less… and so there goes my idea of stuffing a hop up tool into the storage compartment of a vertical grip. Solution? Source new hop up unit that doesn’t require a hop up tool. FCC released their own version of someone else’s idea recently (similar to STTS, I believe) – which features a drum dial adjustable on the fly, without the need for any tools. As it was on sale at, I had a hard time passing it up over Christmas this year. Partially a result of going with this FCC Gen 2 Chamber (this link goes to FCC’s own product page), I made a decision to use a GBBR dust cover I had lying around – this means easier adjustment of the FCC dial, but also means that the dust cover can’t be closed due to the internals in a PTW. Oh well, this upper has already been through a snow day and nothing fried, despite water getting inside everything.


There are a couple of qualms I have about the FCC Gen 2 Chamber – first of which is the fact that I had to shave down the front of my charging handle to allow the dial sufficient clearance to even turn at all. As you can see here, the dial does sit exactly where the front of the charging handle would usually occupy in an assembled upper receiver. No instructions provided out there on this little detail… though plenty of people commenting about “some modification required” and this exact modification being necessary in their PTWs. As you may be able to see, there is already wear on the corners of the dial adjuster from metal-on-metal friction. Even now, the adjuster is still tight (probably rubbing on something), but I’m way past the point of doing guess-and-check to grind down the charging handle to allow for smoother adjustment of the dial; it now works as is, though not as smoothly as it could be.

Featuring many O-rings, the FCC chamber is not nearly as intuitive to assemble as the stock Systema chamber is – but it’s a good thing FCC published a video on assembly instructions to YouTube. I’ve noticed that a couple of the O-rings are already starting to come apart… but I’m fairly certain that these ones don’t serve a terribly functional purpose as is.

Speaking of stupidly designed parts, the spring inside the FCC Advance Hop sets is well documented by many a must-replace item out of the box. There’s more information on this elsewhere on the internet, but my understanding of it is that the Systema cusions (or in this case, Systema-style DIY cushions) are more consistent… or something like that. Not sure why FCC continued to throw in that spring despite many user reports saying that the spring wasn’t as good as the cushions, but that’s not my place to judge. Swapped it immediately and moved on.

Do note that there’s a small bit of wiggle room between FCC chamber and Dytac outer barrel (you can actually see the gap pictured if you look closely) – this does allow the entire hop up chamber and inner barrel to rotate ever so slightly. I haven’t had any feeding issues as a result of this, but it does cause me to wonder if this upper could be more accurate, as the hop up roller has a chance of not being exactly level every shot.



Interesting note – I can now completely dial off any hop up setting with the FCC Gen 2 chamber, MAG Curve Roller, DIY Systema-style cushions – with ease, whereas an otherwise identical set up installed into a Systema chamber would only get me to about 5% hop on (roller visible when looking down bore of barrel) with the hop dialed down as far as it comfortably goes. This potentially does mean that I can use higher FPS cylinders, without the overhop @ 0.30g result that local player “Mouse” reported on the MAG Curve Roller in Systema (Evolution) chamber at ~445 FPS (chrono’ed on 0.20g), even with his hop up backed off all the way.

On a semi-positive note, FCC did include a barrel key that featured a threaded hole for easy removal… but I didn’t have a screw handy that actually fit it. Instead, I just ended up picking up an Action Barrel Key Set for Systema PTW, which included 3 barrel keys and 1 matching bolt to function as a disassembly tool.

Installed in the FCC Advance Gen 2 Chamber is a MAG CNC Curve Roller with the Orga Magnus barrel. Good news is that everything fits and functions as it should, right out of the packaging-  no issues with any of the other components I’m using.


FPS wise, I’m looking at 25-30 FPS drop between 14.5″ Orga 6.13 and 10.5″ Orga 6.13 barrels. I’m seeing a 25-30 FPS drop between “Jester’s” FCC 6.03 and my Orga 6.13 inner barrels. Works out that a black Systema parts-built cylinder running a Systema M110 spring chrono’ed at 355 FPS on 0.20g, with hop off. Side note, I just found out that my PTW will actually gain a couple of FPS by turning my hop on and will max out at a boost of ~25 FPS after a certain point, in both my 14.5″ barrel with Systema chamber, or 10.5″ barrel with FCC Gen 2 chamber.

Field tested on a cold snowy day with Systema cylinder with M110 spring. Shot a couple of guys in the face from 80-100 feet (sorry) right at the start of the day’s games, but couldn’t really see how far my white BB’s were going in those conditions. Won’t know max range or max effective range till my next visit to the field. However, considering how many potentially unsolved issues there are in this upper build, I’m surprised it worked as well as (I think) it did… or at all, for that matter. It does do it’s intended job though – looks awesome and isn’t terribly heavy – and I’m rather proud of it thus far.

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