King Arms M4A1 Velocity Drop/Feeding Problems
 

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.

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)

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.

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?

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

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?

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

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

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.

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

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

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

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.

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

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.

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.

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?

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.

I'm not too worried about it, to be honest. I shone a light down the barrel and it looks fine - no scratches or bulges or anything.

My 249's gone through 3 brass barrels so far, so don't tell me BB's can't mess up barrels lol

Just an update for posterity, in case any other noobs have this problem. I loaded it with .2s (the sample pack, but probably still better quality ammo than Wal-Mart .12s) and it functioned flawlessly. Thanks to everyone who posted in this thread for their patience and input!

EDIT: I may have spoken too soon. I'm still experiencing stoppages every now and again but nowhere near as badly as with the .12s. Maybe these will be fixed with higher-quality ammo.

take your barrel out and try dropping .20s through it by hand and see if they get hung up anywhere

I had similar issues with mine. ended up changing the hop up unit, hop up rubber and nub and the barrel. The original barrel was warped, and the hop up leaked and was causing the misfeeds. I also wasn't getting any hop up on the bbs till I put one of those bridge type nubs in. Not my best gun after all that, but its pretty consistent and reliable.

Dropping BBs manually down the front, are they supposed to fall right out the other side? I experience no stoppages in the barrel itself but the BB gets caught on the red rubber ring (the bucking?) which prevents it from falling straight through.

ts10z: Yeah, I'm not using this gun for serious skirmishing or anything so I don't need it to be a tack driver. I just want to be able to plink with it and maybe play some informal pick-up games every now and again.

If you are dropping them in from the muzzle end then they should be caught on the hop-up rubber. But if the "entry ring" (on the chamber end) of the hop-up rubber being too tight for BBs that can cause jamming.
So your barrel should be no problems now.
Try to figure out where the BBs are stuck when you had a FIRST jam (stop shooting after you found first jam as the later BB will push the jamming one out of position), they should no longer stuck in the barrel (unless the problem is caused by few bad BBs).

It's the flash hider. I remember now. BBs were shattering on it's way out at one point. Try shooting without the hider and see how it performs.

It seems to perform a little better without the flash hider but I still experienced a stoppage after about 20 rounds. This time, the stoppage seemed to originate at the "entry ring" of the chamber, as ccyg8774 described it. Perhaps that particular BB was out of spec?

You may need a new rubber.

I've seen improperly molded rubber before that were too tight on the sealing ring and had to be bored out a little bit because they were causing jams.
Cheaper to get a new rubber though

So what type of rubber would you guys recommend I get, then? I can see different choices like 60°, 70°, 80°, hard type and soft type. I have no idea what any of these descriptors mean and which one would be right for my gun.

PDI-W

Well, I've had good experiences with Maple Leaf buckings and the PDI W-Hold. Many players also recommend a Guarder clear bucking. As for the different degrees and such, the correct bucking to use depends on the FPS and ROF of the gun. Generally you want to use a lower degree for a lower FPS. For example if your gun is shooting around the 360 mark get a 60 to 75 degree 'soft' rubber, likewise if it's shooting over 400 FPS it's best to purchase a 'hard' bucking.

Well, my gun is supposed to be shooting at 450 but last time I chrono'd it, it was shooting at 290, so yeah... Not sure what to choose in this case. :P