Need some help with my sniper project
 

I am using a Marine-M4 metal body for the gun, which I believe is exactly the one Huang is selling...
Yet the body just seems too tight for the PMag...
Ridiculously, I have to use a hammer to send the magazine into the body and it even causes me bigger trouble getting it out. Anyone having the same problem with PMag? I will try other hi-cap mags and see if it works...

And another problems that's killing me is the hop-up... Well, the gun now just doesn't shoot, or sometimes when it does, it bb drops within 3 meters... I am using a KA metal hop-up which doesn't come with the rubber, so I used the rubber inside the G&G stock plastic hop-up. I am also buying a brand-new piece of hop-up and hopefully it works.

Someone know something about these two problems?

the body might just not be of that high quality. but you get what you pay for.

Let me try to help within those problem,

First, What brand of PMAG do you use ? If it's green labe, it should fit most of the guns, I test on G&P, ICS , G&G, King Arm but never try to test on Jing Gong yet so I can't say much about that.
There are two solution to solve your problem, which one you think is better then go for it, 1, you may consider to get different mags and sell that PMAG, 2, you may consider to do some sanding in the mag well to fit the PMAG also Sanding on the PMAG as well since according to what you mention about hammer to get it in, so it would require a lots of sanding and molding, or you may consider to get different body like G&P or King Arm, it would fit 100 %.

Second, You might set your hop up too high, that's why your BBs drop like a few feet from the barrel, set your hop up to zero and test fire if the BBs fly out then you know the issue. I just wonder when you test your gun, Do you test with the PMAG that you hammer in or other mag ?

KND

First of all, thank you for the help.
I sanded the mag already, maybe I should do more, anyway, new mags are coming if they all don't fit, I ll get a new body.

The gun doesn't shoot at any hop-up position, if it occasionally do, the BB drops out... Is this the problem of the rubber? I am trying a new hop-up that comes with stock rubber. It should come in a few days...

Sounds like the metal body you've got sucks... Like I always say, go cheap, expect to pay a whole lot more money.

As for your BB's dropping issue.. I'd guess it's one of three things, A, hop-up rubber isn't installed right / has a tear in it, B, hop up and nozzle aren't sealing properly so your gun has a massive air-leak, and C, Your piston head has come off your piston.

It could be either one of those three things.

Did you happen to open your gearbox?

Well, I didn't opened my gearbox, I didn't have the right size of screw.. yet it's a new one I got from Huang and I am receiving a M150 one in the near future. I guess it's not the piston issue.
I am now inclined to the air leak. I installed the rubber right, but since that's a G&G stock rubber from a plastic hop-up and I am using a KA metal hop-up, there may be some issues.

Maybe the gearbox you have doesn't have the right fitments with the new metal hop up.
Take the hop up out and stick it in the front of the gearbox, and see if it fits not loose.

the air-leak may also be caused by the poor quality metal body, I'd be a liar if I said I haven't seen that happen before.

That is true too of how the gearbox sits along with the hop-up.

Oh.. that's fine for now... actually it's so tight that the rim of the outer barrel can barely touches the body...

finally i got it work... installed my M150 gearbox today with metal hop-up and G&G stock soft rubber from a G&G stock hop-up. Finally see a flying bb. Now I am inclined to suspecting the quality of the JG gearbox

Could have been an air seal issue that you resolved when you put it all back together.