The $100 challenge.....

OK guys, this is what I think could be my problem.  Nothing to do with the banana cylinder, but I will straighten that in due course with my little hydraulic press and wooden blocks with cutouts to match the OD.

With the factory fixed sights the shooting line is correct (despite it masking the target) regardless of the barrel location, hence the reasonable groupings I was achieving when I started.   However, the scope is fixed to the cylinder and I think the slack in the pivot and possibly other factors means the barrel is not locking into the same position each time the gun is loaded, hence the inability to achieve grouping of any sort. I imagine the deviation is both vertical and lateral.

My plan is to fit an adapter rail onto the barrel where the rear sight normally locates.  There's only two fairly shallow 4mm threaded holes to work with but they should be enough for a trial at least.  To that I'll connect the Hawke red dot off the wife's Cometa and see If I can get it to group again.  The red dot will work as it's visible with a huge range of eye relief.  I wouldn't be able to do it with anything else other than a more refined fixed sight setup.

If this proves my point, then some serious work is going be needed to ensure the barrel locks in place true and consistent every time in order to continue with a normal scope setup.

I hope to be able to post some results in the next day or so.

I can see a couple of issues other than the pivot.  I've got some shims so once I've taken it apart I'll look at the pivot as well, but the latching mechanism isn't particularly positive and the breech seal protrudes and is so firm that it also affects the alignment when loaded.

I'm fairly close to getting the scope mounted, just a couple of mods needed to the mounting bracket to allow the barrel to pivot fully and cock.  Should be able to produce some test results today some time.

The pivot and effective detent mechanism is always worth attention on a new break barrel and normally an immediate warranty return if any doubts.

My Nikko Stirling red dot vertical adjustment is normal but windage is the opposite to the markings which I removed to prevent confusion.

Great stuff