It’s a pretty copy paste description. Don’t know what a nylon metal is? It’s not a metal gearbox as OP pointed out, OP also didn’t get any iron sights as it’s listed.
It sounds more like a faulty unit rather than false advertisement on fps thou as AK commented too. I’d say OP should of hit up the store about it’s poor performance ootb but if it was from GBU good luck with that.
It’s the “fire breaker” description that did it for me as a copy paste effort 
It only has to have a pot metal outer barrel for it to get the “nylon and metal” description these days. 
But it’s hardened Nylon handguard What did they hammer forge some Nylon or what didn’t swing hard enough
Anyway hope the OP can get it sorted.
I had to remove a heap of material to get LDT ATM box into a classic army xm177.
First time comment in this forum. This might be the EBG gearbox. Separated gearbox have better polish than the one come with HK 416 blaster ( what your own maybe). I think you can try to hone the lower receiver just like the guy at this video. He got a separate gearbox and fit it in m4 Sijun receiver. For the gel, try ultra hard gel ( the blur one) ?. Hope it helps, this is the name of youtube video:
EBG gearbox, mẫu gear hoàn toàn mới cho súng đạn thạch đến từ hãng ATM
I just bought one tonight from GBU, I’ll see how it goes. All the YouTube videos I could find sold me on it but now I’m a bit worried. On GBU it stated metel receiver and nylon hand guard with steel buffer tube. I’ll update when I get it and test it out.
Honestly your first mistake was probably buying it from GBU… They’re a literal meme around here.
Plus iHobby currently have them for like $390, think they’re the importers fit them.
A mate bought one too recently but can’t test it at home and doesn’t get to go to games often so hasn’t given me feedback on it yet.
Worst case now I put the gearbox in my m4 I suppose
GBU… TacToys 2.0 
Steel buffer tube? I doubt it, most likely alloy… and a full metal receiver with a nylon handguard’s a little odd. 
Still if it’s any good you could always screw a Noveske or some other generic alloy handguard to it. If I had to guess, being unbranded and with that odd mix of nylon and alloy, you’d have to assume these are a bit of a throw together of parts. 
Well it is technically… Considering that the great Pimply One jumped ship from TaccyToys to GBU! 
Not that he’s there anymore as far as I know? … but the legacy of shitty business practices lives on😮💨
Last I saw that cunt was some fucking stupid pay to play game that was dogshit.
Ooops, must remember to disengage farm mouth before forum my bad.
He’s too busy being a world entrepreneur to bother with gel blasters anymore…
Cory, that is… not Maiphut.
Maiphut’s too busy being a farmer. 
Yeah look I’m not sure, the one that I’ve seen reviewed everywhere is also unbranded as it’s a copy of an LDT ATM gearbox, a good copy apparently. The ops jamming issue seems due to his gels. The shell itself well the gearbox is 390$ new so it is what it is. I don’t understand how the gbu one could be worse then ihobby. The ihobby one is technically false advertising aswell, as far as I know there is not hk416d out with the actual LDT ATM, which is listed in ihobbys description of brand. The last review I watch was the exact gun I bought with the exact knock off gearbox and a nylon shell. And she reckons it’s the best AEG she has bought this far, so who knows?? All I know is from what I can find the gearbox is good, it takes normal gen9 m4 mags and it can be swapped into any similar shell. When I get it, it will be used with 7.5mm goats at a indoor field. So I’ll let you all know how it goes.
And when I say that, the only thing she said was that yes it’s nylon, but the weight was decent. She said the shell had good details. But the stock was the cheap generic one (which I happen to have on my current M4 that came with it) so only time will tell. Even AKgelblaster review of it is the same one with the knockoff gearbox. And her video is just called hk416d ATM so it also must be unbranded. They are all the same. And every video or review online looks fine. The only thing is you can’t upgrade the boxes as far as I know. It’s an upper gbbr receiver with electric lower. It also has a mosfet or knock off one. This forum is legit the only place I can find that had something bad to say… Which I found right after I bought it lol 
Also I know you all obviously are experienced with gel blasting and the shops there are, so in no way am I taking what you say as BS. But I’m just gonna be optimistic, because I just can’t find any bad reviews for this gel blaster besides this op. I’m also expecting the receiver to be nylon. Like I said I have a M4 and it’s a metal shell so worse case it’s getting a new gearbox. I really wanted to go gbbr but it seems more hassle than it’s worth. This rifle seems like a good alternative and the blowback looks the best I’ve ever seen for ebb. Plus I play at an in door field and I usually can only go on Friday nights which is 300fps and under so if this thing shoots over 200fps I’ll be happy. As my m4 is currently in need of new orings and only doing 180-230fps which I’ve been playing with fine enough anyways. So I’ll keep you all updated. SORRY ABOUT MASSIVE POST I HAD TO EDIT IN THIS AS I CANT POST MORE THAN 3 TIMES CURRENTLY
I wish to stress that this is not a copy, it’s the actual LDT ATM gearbox and it sucks
If it’s not as described I’ll just take it back and get my money back. I got mine from GBU. Maybe yours is faulty. I hope it is (sorry) but can you not take it back or is the place you got it closed down now?
Edit that sounds ruder than I meant it to be, I’m was just really excited for this gearbox 
and I hope maybe mine isn’t going to be like this. If you get a ldt receiver to try and it fits let us know. I’m definitely keen to get atleast this gearbox going in something even if it means I have to custom make something. I’m dead set on it.
this is one of the really good reviews I saw. It looks identical to me as it’s an unbranded hk416d as well with the alloy orange muzzle with the crappy stock and full nylon body.
I can’t post the link but here is the title
ATM HK416 Gel Blaster EBB AEG Unboxing Review
It’s a Korean woman it should have eng subs at the start of the title aswell. I found it a good watch atleast she goes into detail with it.
I extracted it from its shell destructively as the lower receiver pin is fucked up and has an overtightened double screw on it
Damn well I’ll keep you updated how I go aswell, and you do the same.