At the end of travel on adjusting sleeve for win model 97 trench shotgun

What do I do now. I received the gun with it put together using threads that created a gap between the barrel and receiver. I took it apart and tried to start it correctly and it will not turn in all the way... lacks about 30 degrees from turning in flush. I've messed with the adjustment collar and it was at the end of the teeth to start with. Engaging teeth away from that end just makes the problem worse. How do I fix this? I read that there are shims for this problem but also read that is the shade tree mechanic's way. Any ideas how to fix and where to get the parts?
Thanks in advance.

I take it you mean it's still loose?

Jim, could you call me at 541-868-7493? I can explain it easier over the phone. Thanks

There's actually others on here that will be able to be more help, I think. They just have to see this first. Pics would help though.