I’m planning to build a custom hk416c using various parts and i have some question regarding the build
Can’t speak for the other Sijuns, but I know that my old M4 ran Gen 9 magazines.
I have found that even thou most mags (outside Well and CYMA) that are auppoyto be of gen8 format don’t always fit the same blaster, like I have a LDT HK with a LDT mag, I got a LDT tracer mag on sale and it doesn’t fit the LDT receiver… But it fits my SLR perfectly…I also have a SLR mag that doesn’t sit right in my SLR but does in the LDT…
Just curious but what country are you in? Sijun blasters aren’t typically available here in Aus
They have been here for a while, was ages ago they had them at Donnybrook shop
Oh, I had only seen a couple of the M4’s, otherwise it was all online through places like Zhenduo.
I’m from Vietnam, sijun is a famous brand here, alongside ldt and cyma/jund. But it still bothers me that sijun mags fit well with slr but slr mags don’t fit into sijun body
Ahh fair. Measure the width of the magwell to mags, a lot of the time you might be able to just sand out the sides a bit to get a fit
The sad thing is that the slr mag fits inside the mag well, its just the mag inject-eject mechanism of the sijun body kit is lower than the standard ldt body, so the slr mag must be at a lower position than normal in the magwell, making the t-piece not connect to the mag to feed gel balls 
Ahh it’s one of those ones, CYMA are the same
My questions still remains, sadly
Sijun SJ81-2 M4 Gearbox (monkeemods.com)
If the sijun original gearbox is like the above then at a guess an LDT should fit both being V2 types.
Its the hk416 sijun bodykit, will the ldt gearbox still fit?
If a mag can physically fit into the magwell and the feed tube lines up correctly it’s usually possible to make it work with the mag catch by cutting in a recess wherever it’s required. Some mags are thin enough to use a craft knife for this if you make enough passes.
Measure off your original mag where to put the slot and carefully cut out the shell, clean it up and final tweak it with needle files.
I made Gen 9 mags work in my Ze Hua F2000 by doing this, the mag catch is on the front of the magazine on the blaster but it was simple enough to pull the Gen 9 mag apart and slot it where it was needed.
Had to do this with several different magazine conversions.
The LDT Tracer engagement slot was cut as explained above to fit in the Gen8 style receiver, and also had to grind off the top of the feed tube to allow it to fully engage with the t-piece for memory 
I also filed down many different mag catch levers to raise/lower the catch surface to suit, or even trim the sides as they were different widths to fit the cutout in the receiver body.
Also some were too wide for the receiver, so were cut shorter and then had to file flats on the latch screw end to get it all to fit/work properly.
Some magazines, were opposite to the LDT Tracer mag conversion, where I actually had to glue a small round piece onto the top of the flat topped SKD style mags to fill the gap between the feed tube and the t-piece 
For memory, I think I used to use sections of old nylon outer barrels to get the correct size for the gels to feed into the t-piece nice and snug.
Some differences can be found between different Gen X types of magazines where the ridge that runs along the back of the mag where the feed door was located, had to be filed down slightly to help them fit the receiver well with less resistance when swapping mags.
There’s probably plenty more that I could mention, but that’s just some of the little tricks that allow being able to fit different mags across different model blasters 
Thank you for all your kind instructions of the magazine and gearbox.
But i still have a question:
I’m not familiar with the Sijun brand H&k416… not here in Australia, at any rate.
There are two ways to find out… one would be to spring for the mag and try it.
Other way is to measure up the Sijun mag and compare it to the dimensions of the LTD. Someone will have one they could probably measure up for you. You’d have to compare the profile too, may be slightly different on the corners.