Nambu troubleshooting...

[DavidJayUden]

I'm flying blind here so bear with me.
If it is stovepiping the fired case, might that suggest that the recoil is insufficient to bump the rim hard enough to eject it? Does it feed correctly using live ammo. (or dummies) and cycling the bolt by hand? Also is the ammo. of sufficient velocity go work the system? I understand that there aren't many choices of ammo out there.
Is the ejector long enough to hit the rim early enough?
Have you reverted back to the original mainspring? I'm also wondering if the replacement spring isn't too stiff.
Maybe give us a few more details on what it is, and is not, doing.
dju