Why your KA M4 doesn't assemble properly.
 

After seeing this issue for the millionth time, I decided to go over every millimeter of a king arms M4 that wouldn't close. I found the culprit.

As you probably know already, the lower receiver pins/screws have to go in a stupidly specific order. The part that actually catches and prevents the receivers from mating is the hop up unit. The gearbox sits angled slightly downwards, so the top of the hop up unit hits the top of the gearbox shell and misses the hole ever so slightly, so the receiver can't go any farther forward.

Also, if you find yourself with a surprisingly low FPS after reassembly, take out the buffer tube main screw and the pistol grip screws and replace them with just enough torque to stay put. This will hopefully fix the gearbox's angle and will fix the FPS issue.

Just thought I'd share this, I know it's given me a headache in the past. I'd take a picture, but I'm not taking this damn thing apart again.

Wow, this would explain a few things. Thank you so much for posting this as I am actually just reassembling my KA based M4, and I have ran into this exact problem in the past. It was a pain to say the least and it is good to finally see a fix.

To fix this permanently, you can add layers of aluminum tape (for heating system insulation) inside the receiver, just front of the trigger hole.

It will position the mechbox at the right place to fix theses issues.
It's long to get the perfect results, but once it's done, no more headaches!

I had a different issue when assembling my KA M4.
To put the upper receiver back you need to pull the bolt handle to the back at some point, but that can easily pull the dummy bolt out of its "rail" that allows it to slide back and forth, then the dummy bolt will be out of place, and stays on the back, preventing the upper from going into the place for few milimeters.

I almost always have that happen when I take my upper off. It is a pain but I don't think there is much that can be done. It just seems to be the way the spring pulls the ornament plate that makes it pop out of the guide.

It just takes some messing with to get everything back together.

Just get GBBRs you guys!

My "solution" was to pull the bolt handle as little as possible when I put the upper back. If I do it carefull enough it won't pop out most of the time.

Don't tempt me, that is what I want next. :D

I usually try and do the same. In addition to that I usually told the entire rifle upside down and try to keep a finger on the plate. Once you are over the gearbox it doesn't have nearly as much room to move and usually by that point the charging handle has cleared all it needs to.

The other issue is that you have to remove the bolt release before you can get the mechbox out. Everything else is standard AR.

Damn, I thought this was just me. I also solved the problem by installing the upper before the buffer tube and grip screws.

never had issues getting it assembled, guess I just happen too assemble it right. but the low fps thing might explains mine, I just though it was the cheap China spring I put in it giving out.

Didn't really have an issue re-assembling mine either...hmm. Air seal is okay, I checked.

So is it a KA body issue or mechbox issue?

It's not brand specific...what's been described has been in effect for ages.

When the stock screw is over-tightened it'll pull the mechbox back and potentially cause an airleak. You can't really torque things out of whack too much with the grip...but if it's an ill fitting grip that binds with the contact points of the receiver then it can.

If you want to see how much it can pull it out of whack don't put on the grip or the flora pin and tighten up the stock screw. Odds are that you won't be able to put the flora pin in but the grip will still go on.

Flora pin in first...stock pipe/screw next (just snug)...grip next (just snug)...tighten things up. In reverse for disassembly...then you don't need to pound on your flora pin.

This is super helpful. I have a KA M4 that has has stupid low fps no matter what I've tried. Gonna have go try this tonight.

+1 - almost exactly what I was going to post.

Biggest problem witht he KA M4A1 is the faux bolt coming out of its track - as stated above