Need Help Diagnosing AEG issue

What bbs are you using, bb weight, and what is your compression set up?

 

if you have tried all those combinations of receiver/hop/buckings...
(and I'll assume a couple of barrels & stuff)

I take a stab at something within gearbox - like tappet plate
Doesn't explain why the FIRST 20 bb's are poor
but then corrects itself for 100 or so floowing bb's

I'm wondering if something is impeding the tappet plate moving back n forth smoothly
causing a drop in fps, but then it stops snagging & seals better & proper fps etc...

BUT - you would think one it has cleared, maybe a tight fit or burr - smoothes itself
then it "should" in theory run itself in and not snag or not as much

Tappet could be catching on sector or in runners - tight fit, lack of grease
a way too long tappet fin is another possibility if cycling quickly

I'm stabbing at something up with tappet perhaps
but could be almost anything - heck a cracked spring or something daft
(a bent up too large o-ring, catching at times on a large cylinder port)

think the box is due to cracked open, take some pics & give a little more info
coz there has to be something very strange out of sync or alignment imho

very strange symptoms

 

I fixed a problem on an m4 a while back that was similar though not as pronounced. The problem was the piston o-ring being too small. What was happening though is that because of his shoddy piston lubrication, the o-ring would actually expand after a few shots from the friction warming it a little.
I'm not saying that this is what is happening but it might be worth checking any seals or o-rings that might be likely to expand after warming them up with a few shots. I'd imagine even the packing will expand a bit from the nozzle and the bbs rubbing it.
From a logic perspective, the fact that it starts working after firing it a bit and then stops once it's been sitting for a bit means that it could ONLY be heat. The only thing that would change after 20 or so shots is temperature/friction related. Everything else in a gun is clockwork mechanics and doesn't change unless it is breaking for good. If something changes only with use, it is caused by something external to the gun mechanics and that only leaves the option of heat or in some rare cases dirt buildup. But with dirt buildup you generally won't see a gun return to its previous state after resting for a while. It's most likely heat related (or like guges said, something is expanding and then settling.)
Sorry if my explanation was redundant or repetitive in areas.

 

This is a classic case of "midcap syndrome". This occurs when the feed springs for your magazines are too strong and force BBs against your air nozzle with enough force to move it and cause low FPS and misfeeds.

The easiet remedy for this to trim a few coils off of your tappet spring to increase tension on the tappet and nozzle assembly. Gluing the nozzle in place can also help as well. Trimming is a trial and error process, so only cut a few at a time.

 

I always though of midcap syndrome was more a case of crappy midcaps

You know, them terrible midcaps you bought cheap thinking they would work
Then found out as the spring snaked through tight sharp bends with possible burrs
the bb's would snag & stop feeding, leaving you thinking it was feeding fine...
but then stopped - must be bucking/nozzle issue

change various bits - still no joy
(well no - duhhh, you are still using the SAME crappy midcaps that snag)

Cyma metal midcaps are pretty damn good, the few I have that is (120/190)
wish I just bought those & avoided those cheaper snagging mags I bought

To me, suffering midcap syndrome is encountering feed issues
and foolishly think it can't be the moody midcap coz midcaps are awesome
(not them crappy cheapo midcaps though)

(and more fool me for tearing the box apart without testing on good high caps)

 

I suppose this could be the case but it would be a really easy thing to check. In this case, the reason the gun would fire well after 20 or so shots would be because the mag spring would theoretically lose tension. Try putting like only 30 bbs in the mag and then fire the gun cold. If it works, then there is your problem. If it still doesn't work for the first 20 or so shots then it's pretty unlikely.