M4 firing problems

[Tiercel]

Yes its me and the G&P M4 again!

Right, so after a whole myriad of problems culminating in stripping a gear Ive rebuilt it for the umpteenth time this week. Heres a brief history to help with diagnosis:

- was working fine
- replaced standard barrel with a madbull V2 black python 6.03 barrel in it with guarder clear hop rubber
- was working fine
- took it to a skirmish, where it decided it didnt like a particular brand of BBs and kept constantly jamming and eventually locked up the gearbox
- unlocked GB, bought different brand of BBs that I know work with it
- was working fine
- replaced standard G&P hop unit new ProWin CNC M4 hop unit in it because I lost one of the cogs on the original hop and thought Id kill 2 birds with 1 stone and get a new fancy hop
- was working fine (and putting BBs into orbit around the moon - the range on it was obscene)
- suddenly for no reason a few days later it shit a gear and ripped all the innermost small teeth off the bevel gear
- strip down and replace gear, checked the shimming and everything else, gearbox is working beautifully
- now it wont fire BBs

The gearbox cycles fine both in and out of the gun. The first BB in the chamber will fire and after that it wont fire anymore. The gearbox still cycles and sounds healthy, but nothing comes out the barrel. Taking the magazine out and looking into the feed tube you can see theres a BB sitting in the hop unit, and the nozzle moves back and forth BUT it looks like its *just* not making contact with the BB by about 0.5mm so it doesnt quite push the bb right into the firing position and all the air pressure goes down the feed tube rather than sealing the BB into the hop unit and pewpewing it miles. I've stripped and rebuilt the geabox to see if perhaps something isnt aligning, everything is sitting perfectly where its meant to be, Ive got good compression and good air seals.

Im at a total loss as to what the problem could be now Clearly something somewhere isnt aligning quite right, or pushing forwards far enough... but everything is sitting in the right place.

Anyone got any ideas before I just give in and send it to the local repair shop and throw money at them to figure it out and fix it?

[Sitting Duck]

Re: M4 firing problems

Sounds like a bit wear, odd tiny bit reassembly might have throw stuff out a hairline whisker....
could be bucking lips, might need a bit of tape in magwell fix but yeah nozzle is not clearing just enough to chamber bb's....

But above all it is the usual poxy tappet plate 8mm max window of travel that requires very precise checking to ensure it all works correctly in that tappet window on v2's
V3's often have more tappet travel - say 8.5mm+ which means tappet/nozzle retracts a smidge further which greatly improves the chance of chambering bb's especially at faster speeds or on stock mags that aren't so great at feeding
(no I don't mean it will feed perfectly but should all help if tappet travel is a bit longer)

The tappet travel length is not the only issue, you still need to get the "window" in the right place and a slightly slightly longer/shorter nozzle or even different bucking and hop unit can throw this fine tolerance right out - the wrong side of window and you may get poor feeding as nozzle is smidge short of chambering bb on full retraction or loss in fps as it just barely kisses the bucking and not quite sealing
Other factors come into play and yes believe me different hop units/buckings can really screw it up sometimes
(well that is my take & reason why some people find Madbull and/or prowin hops either great or crap in their guns)

Anyway couple of basic tests......

cycle gun on auto and get it to stop on near full piston retraction to guarantee the nozzle is completely fully forward
(or if gun is running at 10fps then it might stop on semi but we want nozzle 101% fully fowards)
now place hop unit up against front of box and you should "feel" nozzle start touching bucking with say 1mm to go before hop is right up against box
shouldn't have any more than 1mm or you will need loads of retraction of nozzle and likely to wear bucking lips out quickly if smashing f*ck out it with nozzle
Well my guess is that you should feel the seal so to speak as fps isn't your problem but retraction is perhaps stopping your bb's loading

check done - remove hop from box/nozzle test and fire gun as normal on semi blah blah blah.....

bucking test - plop a bb into hop and see how much resistance the lips have....
bb sitting in front of bucking, take a blunt/rounded - well not sharp object and see how much resistance it takes to pop the bb past lips into barrel under nub
you should feel moderate resistance with a pen or cleaning rod end - can't tell exactly how much but it should just need MODERATE pressure to pass into barrel
some buckings if bent or just some in general installed into some hops - even if installed with utmost care & love can give too much resistance and yes this can really impede the bb feeding into barrel. So check that bucking lips are not bent or making it hard for bb's to feed

Phew.....

My guess is that it will all pass those two tests but they should be checked no matter what to rule out stuff
If that is the case then your problem is looking like nozzle not retracting just quite enough...
I presume to nozzle is fully working or appear as it should and a possible delay clip fitted maybe to increase timing and sometimes a whisker more on window size

Next thing you could try as by the sounds of it all the hop and the spring is fully pressed against the box....
Now try to push the hop unit forwards ever so slightly....
either some layers of tape to just slightly push the hop forward or a very thin o-ring over hop/nozzle port to box
we are trying to push the hop just a smidge forwards that will allow the nozzle to clear a whisker more back and chamber a bb better
The trade off with this is that the nozzle may now not quite seal against the bucking so firmly but we are trying to obtain the best position in the tappet window

YES you could file the nozzle but that could be tried later but only file off if you don't lose too much fps
(you can't put the bits back on the nozzle afterwards - hence the test of pushing hop slightly forward first)

Above all - you should try some mags - and possible tape in magwell or on mags first
Honestly before you start modding stuff really really experiment with mags n mag mods to get stuff feeding properly
On some guns they are just right picky bastids others love almost any mags
my 416 is at best a right fussy bitch and needs the mag tape mod to work properly and reduce wobble but most of all needs it to feed properly no matter what stuff I have tried in the past and it also works best so far with cheap red Element G36/prowin plastic hop unit - 2 or 3 others give poor results
(jeez what a pain - this stuff should just work but alas often it never quite works 101% without some minor checks and alterations for picky builds/guns)

Yes check mags and even go back to old or another hop unit first before you start messing with any modding or more work
There are a few more things you could do but hopefully some of this crap above might help you narrow it down and at least get some improvement on the feeding of ya gun

yes filing tappet plate is possible but will not push nozzle further back - only further forward which isn't gonna help as such if nozzle doesn't clear perfectly
however...
filing front of tappet plate to push forward say 0.25, and file nozzle 0.25mm will equate to same sealing but now nozzle clears 0.25mm
but still great care should be used when removing any material hence you should try other stuff and perform a few checks first before you reach for file/sandpaper

[Tiercel]

Re: M4 firing problems

Ughhhh. So before the above was posted, Id been hunting around google for similar problems, and a lot of people having similar problems with AK series guns, and theyd all been solved by filing the front of the tappet plate down to allow the nozzle to be pushed forward further, so thats what Id gone and done.

Made no difference whatsoever, so we can rule that one out.

I did the two first tests you mentioned, they seem fine, nothing out there.

The hop doesnt move at all when the gun is fully assembled, its jammed tight against the front of the gearbox and the barrel spring is either fully compressed or so tightly compressed that I cant get it to move just poking a finger into the mag well.

Im not quite sure what you mean by "mag tape mod" and why that would help, the magazines are feeding fine - they engage with the hopup and feed bb's into it fine, and half a dozen fall out when you remove the mag so theyre definately feeding, theyre just not being pushed fully into the bucking and fired


I put the original hop and barrel back in it, and it fires fine, even with a destroyed hop rubber that had split so I held in place with PTFE (didnt wanna take the current one out of the prowin as its a damn tight fit so why risk it unecessarily).

So the problem is clearly somewhere in the new hop unit... Ive measured them and theyre identical lengths so I really cant figure out why the ProWin has suddenly stopped working? It worked fine for a little while but now its just decided "nope, enoughs enough".

I could go back to using the original barrel and hop, but I begrudge essentially throwing £60 worth of barrel and hop unit away... there has to be SOME way to get these to work

[Tiercel]

Re: M4 firing problems

Some further tinkering:

I had a thought, what if its not the hop unit/nozzle combo? What if the barrel itself isnt coming back far enough in the hop unit to make a good seal?

Results as follows:

original G&P hop + original G&P barrel = works
original G&P hop + madbull barrel = works
Prowin + Madbull = no worky
Prowin + G&P = no worky

So I think weve at least narrowed the problem down to the Prowin hop unit... but what now?

Currently Ive got the madbull barrel and G&P hop in it, it works, but the range leaves a lot to be desired, its nowhere near as good as when the ProWin was working So now I dont know what to do, do I leave it as it is and live with having a working gun and be happy with that, but always knowing it COULD be better? Or perhaps try the madbull ultimate hop and see if that works, maybe Im being naive but youd have thought the madbull hop and barrel are designed to work flawlessly together. Or try to find whats wrong with the prowin? Or try a different hop unit altogether?

Ive also now accidentally shredded my Guarder clear hop rubber because it got jammed in the ProWin like I was worried it would... so Ive gone back to using the old knackered original G&P hop rubber which in comparison seems a lot thinner than the Guarder clear, so Im wondering if that might have been causing the problem with the Rocket BBs as well... so I dug the rockets back out, and nope! Exactly the same problem as when I first started using them, jam jam jam double feed misfire jam double feed. So we can confirm that rockets are shit and just do not work with madbull barrels.

[Sitting Duck]

Re: M4 firing problems

I'm at work atm, but you're local so maybe I can post or lend you some stuff to mess about with

But yeah some hops are different

The hop gets pushed a smidge forward by o-ring or tape between hop n box that allows nozzle gap a little bigger

But no worries I'll pm or explain or post more stuff in morning or PM me as I'm in Croydon too

[ashley0606]

Re: M4 firing problems

Hey Tiercel,

After also just dropping a ProWin in my G&P M4 build, I had a similar problem.

The ProWin went in fine with some small adjustments of the long grub screw (to the front of the unit) but it still didn't quite seat properly. What I did is use the spring from the G&P standard hop and put this over the front of the mentioned Grub Screw this ended up with a firing M4.

Although it may not be the issue it could be worth a shot as the spring isn't a permanent mod and should give the ProWin a good seat over the nozzle

Hope that at least helps


Sent from my iPhone using Tapatalk

[Tiercel]

Re: M4 firing problems

I havent bothered with the grub screw at all on the ProWin, I've heard it's nothing but grief and doesnt work that well so left it out completely. My G&P hop doesnt use the small spring in place of the grub screw, its got an over the barrel spring instead that sits between the hop unit and the back of the M4's outer barrel like this



The ProWin sits totally flush with the gearbox, the small ring on the back sits inside the GB gap perfectly, so I dont think that its not being pushed far back enough

[Sitting Duck]

Re: M4 firing problems

You need nozzle to retract more backwards
Or
The same effect if the hop sat 0.5mm further towards...

The mag feed tube would sit forward and thus nozzle clears the tube a further 0.5mm

You have discovered that not all hops work exactly the same plus add to that factor the M4 hop has about 2mm slack or more that is usually taken up by spring pushing it back...
So the M4 hop is at best a little bit n miss sometimes
G&G often don't use the hop spring on some/most models but can be found on others...
My hunch they see if it needs spring or not

The correct nozzle length is vital and M4's vary from 21.2 to 21.5mm again can make a difference plus some M16 or other variants can be 20.8mm

Add in different hops
Then different tappet plates like SHS blue's, look carefully and the locating lug does not sit 100% centered like on some other tappet plates so nozzle can be sitting say 0.25mm further back than on others

Your nozzle by sanding tappet plate you have just pushed a long nozzle a little further on forward travel

You very likely need to get nozzle further back or shift the hop 0.5mm forward and nozzle will allow a bb to enter hop properly
Don't worry about it getting pushed into barrel coz it will coz it didn't struggle to manually pop a bb through bucking lips

The only possible issue by pushing hop forward a little is if it seals 100% but if you sanded tappet it shouldn't drop or drop very little and if all works better then a light sanding if nozzle will feed better with hop pushed back to its normal position

Think about it and try to imagine nozzle not clearing the feed tube and you should get what I mean or trying to say

I'd post pics but at work with little time to mess about
But you should figure it out but test by pushing hop 0.5mm forwards before filing anything else

- - - Updated - - -

PS don't worry about tape mod on mags or in magwell

It is mainly for get picky mags to work on picky or wobbly magwell/guns like 416 or others

[colinjallen]

Re: M4 firing problems

Depending on the setup, ProWin hops can sit too far back against the gearbox and so need shimming to push them forward slightly.

IMHO, they are terribly over rated.

[Sitting Duck]

Re: M4 firing problems

To throw even more crap & variables into the mix....

Even gearboxes can vary which will affect the hop - gearbox & nozzle retraction or tappet window

Most gearbox shells I've messed with are about 3.1mm at front where hop unit fits
However D-Boys v2 boxes are only 2.65mm thick at front which translates that the nozzle could be 0.45mm further forward than say a G&G gearbox
Hence I found that the Element hop worked best on D-Boys 416 or 800x gun - maybe coz the hop seemed to have the bucking/barrel set a little further foward than some other hop units from "normal" 3mm thick gearboxes/guns

I bloomin' hate these toy guns sometimes

[Tiercel]

Re: M4 firing problems

Ah right I think I see what you mean now! Perhaps the over barrel Spring is pushing it TOO far back and not allowing any slack seeing as it's quite a solid hefty Spring. I was so focused on the nozzle not going forward enough that I didn't consider the opposite possibility. I'll fiddle about with the hop some more and see what happens, perhaps trying the grub screw without any Spring but set to give less rearward pressure than the Spring.

This is all bloody confusing, why can't it just be one in one out?!

As for the ProWin being overrated, it was STUNNING when it worked... Key word being "when" :p seems it's a finicky bugger!

[MasterGouken]

Re: M4 firing problems

I use a cutdown 1911 recoil spring over the inner barrel. It's tight but does just go over it.
Just cut it down to say 1.5cm and put it over the inner barrel and test. If it's too tight against the gearbox trim off a couple of coils and test again until perfect.

Never had much luck with prowin hop's (or madbull ultimate ones either).

[Venares]

Re: M4 firing problems

What hop rubber are you using ?

I only ask as I had a similar issue with a maple leaf where the barrel did not sit far enough back in the bucking causing all sorts of issues.
G&G Green / Prommy Purple no problem.

Personally I'd just stick the G&P back in and either flat hop or R-Hop it.

[Tiercel]

Re: M4 firing problems

What hop rubber are you using ?

I only ask as I had a similar issue with a maple leaf where the barrel did not sit far enough back in the bucking causing all sorts of issues.
G&G Green / Prommy Purple no problem.

Personally I'd just stick the G&P back in and either flat hop or R-Hop it.

Initially Guarder clear silicone, Ive used them for years with various guns and always got good results. Currently the G&P standard rubber the gun came with because I borked the guarder clear one

[sj_asc]

Re: M4 firing problems

I'm willing to bet its the chambers position (too close), or the tappet has wear.