MP9 Misfiring

[JinxDuh]

Picked up an mp9 with a broken disconnecter, replaced it with a Wii-Tech steel one. I have 5 mags, all of them do the same thing. 
 

It’ll fire the first 4/5 bb’s, then it’ll dry fire turning the gun into essentially a bolt action. I noticed that if I remove the bolt assembly and reinsert it some times it’ll go through two whole mags and then the third would do the same as above, fire a few bb’s and then dry fire. 
 

Mp9 is completely new to me, so I’m unsure of what to check. The nozzle seems fine, the feed arm isn’t snapped or broken. Unsure what else to check?!

 

[hitmanNo2]

Are the mags loaded "properly" i.e. neatly double stacked? I seem to recall MP9s can be picky with feeding unless mags are loaded like this. Even down to which side of the follower is touching the first BB.

 

[JinxDuh]

Are the mags loaded "properly" i.e. neatly double stacked? I seem to recall MP9s can be picky with feeding unless mags are loaded like this. Even down to which side of the follower is touching the first BB.

Yeah, I followed Jaeger Precisions video of loading the first bb on the left side, does the same all the way. 

 

[JinxDuh]

Took a slow motion video and you can see the issue. The nozzle returns fine for the first few shots, and then the nozzle and bolt only “half move”



IMG_6961.mov
This is how the bb’s are stacked, with fresh green gas

View attachment 138754

 

[JinxDuh]

Looking at videos yesterday and all of the “issues”, mine doesn’t have the symptoms like nozzle bent etc however I’m unsure if the nozzle is damaged? It looks shorter than say a WE M4 nozzle but google images suggests this is normal. I heard stretching the disconnector spring helps, which I’ve done but I’m not sure if it’s enough. I’m very hesitant of stretching springs for obvious reason. 

View attachment 138790

View attachment 138791

Both nozzle springs also seem fine. A video I watched recommended removing one and testing it, but I’m not sure how viable that is as a fix.

 

[JinxDuh]

One thing I have noticed is that the hop up doesn’t seem to be adjusting, it’s constantly showing as “hop off” regardless of where it’s rotated. So that will be the next thing I will replace and hope it solves the problem. 

 

[Ronin0033]

Can’t say I’ve seen that on any of mine so far, but things to check:

Piston head: make sure it’s not damaged/deteriorating. May be the issue with gas leaking behind it so it’s getting excessive cooldown and only doing a few “full power” shots. 
 

Make sure the pins on rear of bolt are not backing out and slowing the bolt travel. Had this on one of mine when full-autoing. Also chewed my trigger bar with this…

Double check your hammer isn’t broken at the shaft. May look ok when in the gun, but you can only really tell when you take it out and it’s in 2 pieces instead of 1. 
 

As for the hop, it’s either been reassembled incorrectly, or is missing the tiny little bb that applies pressure to hop rubber. KWA hops are somewhat… fiddly. I’d recommend getting a TNT retrofit kit for the MP9, drop in and you can hop a .4 and more…

 

[JinxDuh]

reassembled incorrectly

I reassembled it a few times but still no luck. It has a ball bearing, not sure if correct but I like the idea of the TnT kit so definitely on the cards. 
 

Double check your hammer isn’t broken at the shaft

Had the hammer out earlier and can’t see any issues. I made an effort of taking every piece out, cleaned, regreased and reassembled.  Gave it a test and it got through about 25 bb’s, but didn’t get the chance to further check yet

 

[Ronin0033]

Hmmmm, ok, last things I can think of:

Piston head, check its the correct size for the nozzle. MP9’s have 2 different size pistons/nozzles. If it’s been ever been replaced it could be the wrong one for the nozzle. 
More than likely it’s a green piston cup, with a thicker walled nozzle. But they also do an orange version with a thin walled nozzle. 
(you can slightly squish the thin one with your fingers, the thick one is much more difficult). 
It’s 1mm difference between the 2, so can lead to issues if it’s not correct. 

The only other thing I can think of for the issue of only firing 4-5 shots is the brand of BB’s you are using. 
I had an issue using Excel’s in my mags, they are ever so slightly smaller so were jamming after a few shots on the metal mag lip where it joins the base of the mag. Rectified by filing down the mag lip, or using different BB’s. 

I also just take any sharp edges off the rest of the mag lip to aid feeding, I have the WiiTech metal nozzle in mine and unless everything is smooth it can damage BB’s during the pick up. 

 

[JinxDuh]

But they also do an orange version with a thin walled nozzle. 

This is the one in the pew currently. 

or using different BB’s

I'll give this a try, as I was using Spitfire 0.25g BB's which aren't great at best anyway.

 

[JinxDuh]

Tried different brand BB’s, multiple in fact. Specna, nuprol rzr, g&g and all same result. 
 

I seem to be getting more shots out before the inevitable issue now though, which leads me to believe something I’ve done has helped somewhere ?

 

[JinxDuh]

I think I’ve found the cause. The magazine is losing gas mid way through. When it starts to half cycle: I took the magazine out, regassed it and it fed the other half of the magazine. 
 

I’m going to look at replacing the nozzle with the thicker wall and green part (Worst case scenario, I end up with a spare nozzle)