Just to be clear, I'm the gun doc that worked on this gun. Only thing changed was the spring that the OP brought in, and everything thing else is stock. Yes, everything was put back together properly and it's fully functional. The issue could be caused by the stock bucking or hop up chamber, or even the spring that was provided. Nothing else has been touched so this does need looking into.

And suggestions has been made to upgrade some of the compression parts due to the low FPS, so this wasn't my doing if that's what you guys are thinking.

Hey, Ghost. Yes, I remember you did suggest that I upgrade the compression parts but I figured that a stock gun fresh from the factory would be able to shoot fine, so I didn't bother. Also, I certainly don't think these issues are any fault of yours because all you did was a rear-wiring job and didn't touch any of the other parts.

Stock/factory parts doesn't mean good quality. That King Arms M4 is King Arms entry level gun so the internals are a hit or miss. From what I remember, there was a good reason why I suggested on replacing the compression parts as they didn't seal properly and leaked. As to the misfeed, that may be caused by your mags as it was feeding and shooting every shot with my mags and bbs. Because your gun was never used and tested, we don't know if the issue was before or after my work. Either way, you can bring it by so I can diagnose the issues.

Okay, I'll bring it by sometime in the new year for you to take a look at. I'll send you a PM next time I plan to be in your area. I tried shooting it again just now and aside from the prior misfeeds/double feeds, it spat out broken chunks of BB, so I think there's something seriously wrong with it.

yea sure. What bbs are you using? Try pushing up on the mag when firing and see if that helps. Also try pushing up and forward and/or back. Make sure the hop up setting is completely off (full turn CCW). Shine a light down the hop up and look down the barrel. Make sure there's no obstructions or anything else.

So here's a funny thing. When I was breaking the gun down to get a look at the hop-up, I found this in my flash hider:

http://i.imgur.com/qkQS7VO.jpg

Now I know where the broken BBs shards were coming from! Also, when I stuck a cleaning rod down the barrel, no less than 17 BBs came out. I guess the internals were leaking SO much air that there wasn't enough velocity for the BBs to exit the barrel and they were just getting packed in one after the other. You had the same thing happen when you test fired the gun, if I recall. In light of this, I don't think I'll be shooting the gun again until after you've had a look at it. :P I also think this rules out magazine feeding issues as it wasn't misfeeding, the BBs were just getting stuck before they came out the end.

As for the BBs, I don't even know what brand they are. I got 20,000 rounds for $15 or something like that at Wal-Mart. Definitely not high-quality ammo to be sure.

That is most likely the problem, not air leak, not the velocity being too low. Most Walmart grade BBs are meant for Walmart clear body springers, and they are known to have issues in AEGs.
Rough surface, bad molding line are common on poor quality BBs and that causes jams.

They feed and perform fine through my Classic Army G36C, though...

DAMNIT MAN THOSE ARE .12s!!!
Throw them all in the garbage right now, all .12s suffer extremely poor quality control and are well known for messing up airsoft guns.
Depending on the level of damage, you may have to replace your inner barrel now too.

Sometimes BBs are bad, not because all the BBs in one bag are bad, but a portion of the BBs are bad, and that is enough to cause problems. Actually, just one bad BB jamming in the barrel is enough to make later BBs jam and jam 17 or more in a roll. It could be your G36C get the slightly better BBs in the bag, or your G36C has a widerbore barrel, or both.

BB jamming in Hop-up or not feeding from magazines could be several reasons, but when they stuck in the barrel... It is generally bad/dirty BB or bad/dirty barrel, in this case, STARTED with the BBs since your barrel WAS new. (Your inner barrel may be bad now as the BBs scratch it.)

The BBs are supposed to be able to slide down the barrel freely, with no pushing and no FPS. You may have a airseal issue but that's not why the BBs are stucked.

Yikes! Okay, I'll discontinue use of the .12s immediately and pick up some .2s. I should also avoid the sample pack of .2s that came with my gun, correct?

They are generally not good but normally the sample pack will not harm your gun.
But there was once I found a CYMA gun came with a small pack of Golden Ball 0.2s as the sample BB, they are pretty good. Not as good as BB bastard or King Arms BB but reasonably good.
There was also once I found the "0.2g BBs" came with a TM EBB pistol (side notes: TM EBB pistols are fun but useless) are actually 0.12g. (They are good 0.12s, but why would TM lable them as 0.2?)

If you're playing outdoors, and your fps is in the 365-400 range, use .28s
below that .25s
indoors .20s

When I first decided to try and get the gun working, I only intended to use it for plinking in my basement so I didn't think the lower velocity and cheap BBs would be much of an issue. I certainly didn't think the stock parts wouldn't even let me hit something 20 feet away! Depending on how much of the internals need to be replaced and how much it would cost for parts, I figure I might just retire this gun and let it serve out the rest of its life as the photography prop it was meant to be. In the meantime, I'll try loading it with .2s and see what happens. If I'm lucky, it was just the crap BBs causing the problem.

Oh, and happy new year, everybody!

Inner barrel? That's a little extreme. Shitty plastic BBs vs metal barrel, metal wins.

He might have to replace his hopup rubber but that's about it.