Moving M4 HopUp mating issue
 

So ive come across a strange mating issue with my gearbox and hop up.

If I push the hopup manually against the gearbox I get about 400fps, with a mag in about 250fps.

Now what ive noticed is when a mag is put in, the mag will literally move the hopup about 1-2mm away from the GB which I assume causes the airleak issue. Problem is it seems different mags move the hopup different amounts!

Now my question is what would be the best way to go about fixing this problem?

Here are some pics:

NO MAG
https://s31.postimg.org/u9tgvmtuj/IM...804_204222.jpg

WITH MAG
https://s31.postimg.org/91fse7fdn/IM...804_204312.jpg

Hunterseeker Armouries barrel lockdown kit. thank me later.

if the hop chamber is incompatible with the gun, locking it down will only break it when the mag tries to force it into position

This. Does your hop unit got the spring on the front? What about the rubber installation, or the condition of the nozzle? To see a drop in FPS that much I'd also check to see if your hop housing is cracked...

Hop up does have the spring in front, this is the hop up which originally came with the gun. Rubber is good and nozzle looks practically new.

I have tested the Hopup, rubber and barrel assembly on a different gun and it works perfectly yielding the exact same FPS as the assembly the gun normally is with.

One thing I have noticed the nozzle is it seems to be just a smidge longer then my other M4, as when I press the barrel assembly up against the gearbox and drop a BB in, it wont feed the BB.
I have to push the hopup forward a hair bit before it will drop in place.

I only noticed this issue now after acquiring new mags, and have attempted to test FPS for selling purpose.

Note: the massive FPS drop is when the hopup is pushed forward as far as I can get it. Generally speaking I can get it to range between 300-380FPS by pushing it forward by different amounts

If you wiggle the mag, does the hop chamber at some point rest on the mechbox?

Not particularly no. Each mag pushes the hopup forward a different amount and kinda just keeps it there.

Socom Gear mag = ~220-250FPS (Seems to push the furthest)
CYMA Mag = ~250-280FPS
ICS Hicap = ~380FPS (Barely pushes it forward)

I remember originally using Elite force mags with and swore that it shot 400FPS...so maybe im not going crazy? haha

Some orings to hold the barrel back against the gearbox but still have give to allow rounds to chamber, even a stiffer spring perhaps.

Basically, limit the play and but in turn limit the mags that will work with it eh? Hmmmm it just might be the ticket.