UPDATE:
Replaced the pins and springs in the bolt as well as the mainspring in the tang, no improvement at all...BUT........
We compared things side by side with another 336 about the same vintage and think we've found the problem, but don't know how to fix it; he will probably send it to the factory at this point.
When you operate the lever so as to chamber a round, the bolt does not stay locked fully forward to the chamber. It acts like there is a spring between the bolt and the chamber and it pushes the bolt itself backward, I'd guess at least 20 thou.
With the hammer all the way back in the firing position, if you hold the hammer to keep it from falling quickly but let it fall all the way down, then push on the hammer (and thus against the bolt) with your thumb you can push the whole bolt assembly forward that +/- 20 thou and push it up tight to the chamber again. Even holding the lever tight to the grip, the bolt wants to push back on its own. The one we were comparing to has no such tendency no matter what we try.
I'm not comfortable going any further than I have as far as repairs so I think sending it back makes the most sense.
Just thought you'd like to know what we found, even if we didn't solve the problem.
