Up until today, my Zenith MP5PDW has been pretty flawless.

I replaced the stock trigger housing group for a semi auto ambi navy one from HKParts.

I hadn’t shot it in a while, but took it to a match today and had nothing but issues.

I could shoot 1,2 and sometimes 3 rounds before the trigger went dead. I could pull it, but it felt like it wasn’t engaging anything.  I would rack out the good round, and it would fire once more....then dead trigger again.

I have isolated the problem to the trigger group.

I have been looking at this trigger mechanism inside and out. The best I can tell, is the hammer may not be correctly “timed”. The hammer appears to sit rather high in the trigger pack.  The trigger cocks and releases just fine when not pinned to the receiver...albeit a little mushy. When I push down partly on the sear(simulating what I think is happening when attached to the receiver) and try to pull the trigger, I get the same “dead trigger” feel.

I can see that when I push down that bit on the hammer, the sear slides forward...resulting in the rear of the trigger unable to trip the rear of the sear anymore.

What do you guys think is going on....can you confirm what I am thinking?, and how can I fix it?

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The old housing is a non-ambi so I can’t swap. I had purchased the entire new trigger group complete from HK parts.

I did look at my other stock unit and compared it to the new one. Functionally it appears the same.

My hypothesis is that the hammer is being held down just far enough by the bolt carrier that the sear disengages the trigger.

Noobie need your help!

Here’s mine



I pushed my hammer to the same position as your pics and had the same results you did.

I’d disassemble it, clean it and see if it’s continues.

My guess is a spring is installed wrong.

I checked over my springs. I look at the sear spring? Trigger return spring? The one at the front.

I thought...that doesn’t look right. That spring isn’t  designed to work like it was installed.

After doing some research, this was incorrectly assembled by HK parts. I took it apart to find the spring stretched out. This would explain why it functioned ok in the beginning, then just now starting to cause failures.

My earlier hypothesis was incorrect as I discovered prior finding this spring issue, that the hammer is not under tension when the firearm is in battery.

It was a pain in the dick to reassemble the trigger pack with that stretched out spring, but I finally got it. Hope that does the trick.

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I went to the range. It didn’t work.

Now I’m gonna take it apart.

This is what I fear the most about these platforms. If something doesn't work, I'm screwed. I'm so mechanically retarded, it's not even funny.

After 10 years, I feel comfortable with most all AR issues, but I can't shake this mp5 bug!

Hope you get the issue figured out.

Are you able to duplicate the issue at home by manually manipulating the controls?  It sounds as if your bolt isn't pushing back enough to re cock the trigger system after firing.  Have you tried differn't ammo?  Have you put the OEM trigger pack back in to see if issue clears up?  Also check your roller locks. Make sure they are moving freely.

Check to see if the elbow spring is installed correctly.

So after reading a few forums about trigger issues, I decided to try swapping my sear and hammer from the original Zenith pack into the New Ambi pack from HK parts.

I immediately noticed the difference on fire and reset.

I went to the range and everything is working great!  I ran it suppressed and unsuppressed, used many varieties of handloads, bullet weights, factory ammo, etc. everything functioned just fine with the 80 degree locking piece. Even my weak hand loaded subs without the suppressor ran fine. Ejection was certainly weaker, but it all ejected just great.

All is good again. Must have been some part tolerance issues or a bad part.

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