Fred,
Here's what happened, as best as I can recall (it was 3 days ago, and my memory is full of lots of stuff these days).
Acquisition of 30/30 barrel and receiver (call it frame 'A') that are factory mated, plus one 357 barrel that I understood 'fit' the same frame. My idea was to add the 357 to another frame (call it 'B') that I have. When I put the 357 on my 'B' frame, it sounded and felt like it latched, but when I pulled on the barrel (moderately hard - with no forend) it snapped open.
Then I put the 357 on the 'A' frame since I believed it already fit that frame, and it seemed to latch up fine without a forend. Pulling did not cause it to snap open. I then added the forend to it on the 'A' frame and it snapped open when I pulled moderately on it.
I then put the 30/30 barrel on the 'B' frame and it fit and snapped into place with, or without a forend. And it did not snap open with pulling either way.
So the 30/30 barrel was okay on both frames. I decided to put the original 30/30 back on the original 'A' frame, leave it alone and work on getting the 357 together with the 'B' frame.
Back to the 357 on the 'B' frame, which snapped open when pulled very hard without a forend, and relatively easy when the forend was installed. I tried all the forends that I had, and even changed some buttstock (don't ask why

I was just trying everything)
That is when I posted the original of this thread.
After getting the responses, I did discover that the stud on the barrel and the forend hole(s) did not line up exactly and I filed one of the wooden forends so that it would fit better. That seemed to help initially, but when I pulled really hard on the barrel it still popped open. (Before filing, it popped open easily, so I figured I was on the right track.)
That is also when I went back and read about looking for an indication of unever wear on the shelf. It appeared to me that I had that, so I did the 'file and try' method until the barrel did not pop open even with lots of pulling pressure. And that is how it is still, the 357 barrel is on the 'B' frame and does not pop open.
Fred, I know you are very good at this stuff, and are very precise. But I am just a basic tinkerer with basic set of tools in a garage, so I am wondering if I might be still missing something, or if I otherwise screwed up?
Kerry