No bb firing when front receiver pin is in

[Pollynator_bravo2]

Afternoon. I’ve been putting together a m4 from a Specna arms aly reciever (upper&lower) and a gate eon gear box. I have a maxx m4a hop unit inside. Gear box is shimmed well and cycling nice a quite. Nozzle is 20.5 mm I think. I measured it now can’t remember the figure doh

Anyway. No matter what mag I put in it when the front reciever pin is in nothing comes out when the gun cycles. Hop is wound right off. I took the delay chip out no difference and i put it back still same. The pin is stiff going in I wonder if at some point it got mixed up with another one but im 99% sure it hasn’t. I found some spacers behind the infeed adapter on the hop unit so I took them out and no difference. I held the upper in place and fired a few rounds and most came out a decent pace but couldn’t chrono. Any ideas as this one has me stumpers. The outer barrel has a M lok Handgaurd I’d not fitted one of these before so not sure if it’s right or wrong but it all goes together ok and I looks like the eyes of the front receiver are aligned

 

[Anonymoose]

Nozzle length?

 

[Pollynator_bravo2]

Nozzle length?

Click to shrink...

20.5 from memory but I’m about to rip it all apart and measure literally everything lol

 

[Adolf Hamster]

Does sound like nozzle length, assuming it's aftermarket then you want to check it against oem.

Certain hop rubbers (particularly maple leaf) in *some* guns the feedlips can protrude back into the hop chamber and acheive a similar effect. My go to for such cases was the pdi w-hop with standard nub.

Otherwise, how are you securing the hop to the gb? On an m4 you typically want a spring pushing the hop towards the gearbox, although a bunch of o-rings round the inner barrel can make for a more solid relationship. Either way you dont want the hop sitting too far forwards. That said the typical symptom here is it fires ok but with dog shit consistency due to the nozzle-hop leaking.

Of course this all assumes good mags and good ammo, if i had a penny for every misfeeding gun i could "fix" by handing the owner a magfull of geogfs.

 

[Pollynator_bravo2]

Does sound like nozzle length, assuming it's aftermarket then you want to check it against oem.

Certain hop rubbers (particularly maple leaf) in *some* guns the feedlips can protrude back into the hop chamber and acheive a similar effect. My go to for such cases was the pdi w-hop with standard nub.

Otherwise, how are you securing the hop to the gb? On an m4 you typically want a spring pushing the hop towards the gearbox, although a bunch of o-rings round the inner barrel can make for a more solid relationship. Either way you dont want the hop sitting too far forwards. That said the typical symptom here is it fires ok but with dog shit consistency due to the nozzle-hop leaking.

Of course this all assumes good mags and good ammo, if i had a penny for every misfeeding gun i could "fix" by handing the owner a magfull of geogfs.

Click to shrink...

Geoffs ammo and I’ve tried a mid cap PTS, Krytac, G and G and even a shitty hi cap nuprol.
The nozzle appears to be going back and forward and reach the lips ok that’s what stumped me as I’d suspect if it was then it would fire but the bb would come out like something on a matrix film

 

[Nick G]

Does it fire if you drop a BB dirsectly into the feed tube ? Could be a misalignment issue .

 

[Pollynator_bravo2]

When I hold the upper receiver in place by hand it’s firing so is the nozzle now to long?

It’s held back by a spring

 

[Pollynator_bravo2]

Does it fire if you drop a BB dirsectly into the feed tube ? Could be a misalignment issue .

Click to shrink...

I tried this yesterday there was two in the and one shot out the feed tube which was impressive being a maxx and they normally lock in place

 

[Nick G]

If it feeds without the body pin in and when you drop a BB in then my money is on something becoming misaligned when the pin pulls thr top reciever down tight . Try a different hop unit and see if that gives you more clearance .

 

[Adolf Hamster]

Geoffs ammo and I’ve tried a mid cap PTS, Krytac, G and G and even a shitty hi cap nuprol.
The nozzle appears to be going back and forward and reach the lips ok that’s what stumped me as I’d suspect if it was then it would fire but the bb would come out like something on a matrix film

Click to shrink...

if the nozzle is too long the issue is that when it retracts it isn't far enough back for enough time for a bb to be brought up to the firing line.

When I hold the upper receiver in place by hand it’s firing so is the nozzle now to long?

Click to shrink...

you holding it back against the pin? does it feel like when you're pushing the pin into place that the reciever is moving even further back?

 

[Pollynator_bravo2]

If it feeds without the body pin in and when you drop a BB in then my money is on something becoming misaligned when the pin pulls thr top reciever down tight . Try a different hop unit and see if that gives you more clearance .

Click to shrink...

I feared this might be the case tbh. When I hold it in place with no pin it fires

 

[Pollynator_bravo2]

if the nozzle is too long the issue is that when it retracts it isn't far enough back for enough time for a bb to be brought up to the firing line.



you holding it back against the pin? does it feel like when you're pushing the pin into place that the reciever is moving even further back?

Click to shrink...

When I put the pin in I can’t do it without tapping it in but I can’t see why. Doesn’t make any sense

 

[Pollynator_bravo2]

 

[Pollynator_bravo2]

 

[Adolf Hamster]

When I put the pin in I can’t do it without tapping it in but I can’t see why. Doesn’t make any sense

Click to shrink...

Pushing the hop unit up tight to the gearbox, which to a certain extend is exactly what you want. Indeed using o rings round the outer barrel rather than a spring the purpose is to acheive exactly that and even to the point where the pin needs a little tapping to get it in/out.

Does double duty of keeping the hop secure and preventing upper/lower reciever wobble.

That said, there is such a thing as too tight.

Looking at those photos, looks like the nozzle is dragging on the inside of the hop? If thats a new development and not previous damage worth considering that could jam the nozzle and prevent firing

 

[Pollynator_bravo2]

Pushing the hop unit up tight to the gearbox, which to a certain extend is exactly what you want. Indeed using o rings round the outer barrel rather than a spring the purpose is to acheive exactly that and even to the point where the pin needs a little tapping to get it in/out.

Does double duty of keeping the hop secure and preventing upper/lower reciever wobble.

That said, there is such a thing as too tight.

Looking at those photos, looks like the nozzle is dragging on the inside of the hop? If thats a new development and not previous damage worth considering that could jam the nozzle and prevent firing

Click to shrink...

Yeah I think there is some investigating to do.

 

[Adolf Hamster]

Yeah I think there is some investigating to do.

Click to shrink...

a test is to assemble the upper+lower without the inner barrel/hop assembly see if it's still tight, will tell you if the hob is being jammed in there or if the gun is just a bit tight.