Stoeger M3K inertia problem?

[CooperG]

Hi all, given that the Stoeger M3K uses the patented Benelli inertia system, I’m hoping someone can help me with a shell cycling problem. 
The gun is a 10+1 capacity 12 gauge that is intended for practical shotgun use, in my case. 
When shooting targets the bolt sometimes closes leaving the next shell from the magazine tube sat on the lifter, even though it ejected the empty. It’s random, different loads, shoulders and stance don’t make a difference. It had a deep clean from new as recommended and even though I’ve recleaned it many times down to component level it’s still a problem. 
Any experience of this and what may cause it please?

I can’t rely on it during a match while it’s malfunctioning. 
 

Thanks. 

[Vertigofirearms]

So the gun is sluggish?

[CooperG]

1 hour ago, Vertigofirearms said:

So the gun is sluggish?

Thanks, I don’t really know how to tell if it is sluggish, we’re trying slow motion video to see if it captures anything relevant. 
Action bar, inertia and magazine springs all good. 
Checking the lifter spring, most likely exchange a few parts with wither TTI or Benelli. 

[Vertigofirearms]

are you saying you shoot the gun then it cycles - but nothing goes into the chamber? the shell stays on the lifter?

[CooperG]

3 minutes ago, Vertigofirearms said:

are you saying you shoot the gun then it cycles - but nothing goes into the chamber? the shell stays on the lifter?

Yes correct, randomly

[Vertigofirearms]

Are you sure all the parts are in the trigger group? did you modify it?

[Steve Rose]

Has it always had this problem, or did it start after some period of working well?

 

Which brand and type of shells are you using ? What is the shell's velocity and shot weight?

 

Have you ever removed and checked the inertia spring inside the bolt carrier? (There was a run of Stoeger inertia springs a few years ago that would break)

[CooperG]

7 hours ago, Vertigofirearms said:

Are you sure all the parts are in the trigger group? did you modify it?

It works a lot of the time and I haven’t seen any problems in the group, will check. 
No modifications as I wanted a factory spec 10+1 with no need to change anything. 

[CooperG]

6 hours ago, Steve Rose said:

Has it always had this problem, or did it start after some period of working well?

 

Which brand and type of shells are you using ? What is the shell's velocity and shot weight?

 

Have you ever removed and checked the inertia spring inside the bolt carrier? (There was a run of Stoeger inertia springs a few years ago that would break)

I don’t recall it jamming or misfeeding apart from a couple of stove pipes when ejecting. 
 

I use a range of shells, Eley, Geco, RC, Joker to name a few. This is more to try and isolate the problem. I’m also trying 67 and 70mm variants. Anything from 28x7.5 to 32x5 load and slug

ive stripped the bolt down and the inertia spring is intact, 

Thanks