King Arms M4A1 Velocity Drop/Feeding Problems

[mc_tammer]

Hey, all! I'm hoping somebody here can help me diagnose this problem. There are multiple stages so I'll list them one by one.

1) The gun doesn't fire a BB every time. For every 10 times I pull the trigger, maybe 4 or 5 BBs are fired. The motor turns and the piston cycles but no BB comes out. It happens regardless of whether the gun is set to Semi or Full Auto. I thought it might be a magazine feeding problem but the same thing happens regardless of whether I'm using the stock HiCap mag or spring-fed 75-round MidCap PTS EMags.

2) I can't hit a target 20 feet away. I set up an 8.5x11 sheet of paper with a half-inch circular point of aim in the middle and I can't hit it! When the gun DOES fire, the BBs begin to arc down almost immediately and impact the target stand and never the target itself. I should note that the hop-up is set to max.

3)The muzzle velocity is way low for my spring. The gun supposedly came with a 450FPS-rated spring but the last time I brought it to a gun doc for some wiring work, he chrono'd the gun afterwards and it was only shooting at around 290FPS. We figured it was maybe just the winter cold affecting the gun when I drove with it in my car trunk, or that the gun came with a lower-rated spring than I'd originally thought. However, when I tested it today, the gun had been in my house for weeks untouched so it can't be the cold, and I don't think the gun would have come with a spring so weak that it can barely propel a BB out the barrel.

Anyone have an idea as to what's wrong? I'd really appreciate any thoughts you may have regarding this.

[Stealth]

LOL first off get a new gundoc. Why would he/she release a gun without verifying that it works as expected?

Onto your issues:
1) Nozzle is probably too long; tappet plate is off-spec; the hop-up unit is not pressed against the mechbox; bad mags; or a combination of the above.
2) Flash-hider could be obstructing the path of BBs; inner barrel length is shorter than outer barre length; bad ammo; very dirty barrel; or some combination of the above
3) air seal issues (see #1)

[ThunderCactus]

definitely new gunsmith.
Take a pick from the asc gundoc listing!
Either air nozzle air leak or piston head O-ring is dried out. Also seen cylinder head O-ring issues.

[mc_tammer]

Thanks for the replies so far, guys! As for my gun doc, I actually found him through the ASC listings, and he usually does great work! I don't think he did anything wrong here.

I should probably mention that I originally bought the gun maybe two years ago, intending to only use it as a photo prop. I only recently decided to buy a battery and shot it for the first time today. It had been sitting dormant in my room until now.

I'll try cleaning out the barrel to get rid of any dust or whatever that may have accumulated over the years, but as ThunderCactus mentioned, maybe the O-rings dried out. My gun doc also mentioned "compression problems" as a possible cause for the 290FPS reading but I don't know enough about the internal workings of airsoft guns to understand what he meant.

Lastly, I just remembered that when I first got the gun, I tried hooking it up to a 7.2V NiMH that I got with a clearsoft gun. I heard the motor move but it wasn't powerful enough to actually cycle the gun. Is it possible that by doing this and then leaving the gun "half cocked" as it were, could have damaged the spring or the internals in some way so as to cause the types of problems I'm seeing?

[ThunderCactus]

no, you cant wear down a spring more than a few fps leaving it fully compressed for a year
7.2 is definitely not enough
aaaand you need to do;
mechbox compression test (with the air nozzle in the forward position)
and a hop chamber to air nozzle seal inspection

Make sure you're not hemorrhaging air anywhere

[mc_tammer]

Regarding the compression test, I seem to remember the gunsmith pressing the mechbox against the workbench a few times and what I'm guessing is the air nozzle would retract into the mechbox and then pop back out when the pressure was released. It's not doing that anymore when I try it. Is this the cause of the problem?

[ThunderCactus]

most likely your mechbox just stopped the cycle with the air nozzle in the rear position, no big deal
BUT if the cylinder head tube that holds the air nozzle was cracked or broken, it would definitely cause all the above problems...
Cycle the mechbox a bit so the air nozzle is visibly in a forward position, and see if you can wiggle it up an down with little force, it should not wiggle more than .5mm or so

[Adamlxlx]

Sounds like your gun doc forgot to put the tappet spring back in.

[mc_tammer]

Thanks for your patience, everyone. It's no secret that I only know enough about airsoft guns to break them down and replace cosmetic parts. That being said, is the nozzle here in the forward position? Even if it isn't, I can wiggle it a little bit (about a half millimeter, like ThunderCactus said). This is the position it was in when I tried to compress it against my desk top.



Adamlxlx: If that's the case, would it cause all three of the problem's I'm having or just one or two?

[ThunderCactus]

that's the full rear position
if it doesn't move back and forward when you cycle the gun then it's likely the tappet plate is broken or stuck, or the spring isn't in there which would also cause that
If you can pull the nozzle out by hand, tappet plate is either broken or the spring is missing
Should come out another 6mm or so

[mc_tammer]

When I cycle the gun, the nozzle does come forward, then snaps back into the position you see above. I can't pull it forward by hand, though.

[ThunderCactus]

k good, so that works.
When it's all the way forward does it wiggle just a little as well?

[mc_tammer]

I can't tell you that because it's only in the forward position for a fraction of a second before returning to rest in the rear position. It's almost like the gun's cycling in reverse. The nozzle starts in the rear position, then I pull the trigger and it pops out and emits a blast of air, then immediately returns to the rear position.

[pestobanana]

That's what it's supposed to be doing. Cycle a few times on full auto or try a shorter trigger pull and you'll have it stop in the forward position.

[mc_tammer]

Ah, okay. So I got it to stop in the forward position and yes, it still wiggles a little bit. This is good news, right?