Since making my original post I have found out some interesting facts & information about air rifle pellet ballistics. First of all there has been little work carried out on pellet ballistics, most work has been done on bullets / shells etc.
However there have been tests and investigations carried out on pellet ballistics (I have included a link to a web site below - the section on PELLET FLIGHT is very interesting). This seems to sugest that the "weight distribution" within the pellet is of considerable importance to the flight, and therefore the accuracy of the pellet, i.e. the center of gravity position of the pellet.
Those of us who build and fly model aircraft (like me) understand that C of G (center of gravity - balance point) is very important to flight.
1) If the C of G is too far "forward", flight is slugish and airplane does not want to respond to controls
2) If the C of G is too far "back" , flight is very, very twitchy indeed, the mearest touch of the controls results in excessive reactions
3) If the C of G is too far "to one side", flight will always want to turn to the side which has more weight (left or right)
Sorry to go on about aeroplanes, but the same C of G principle I would think, must apply to all types of flight (aeroplanes, helecopters, pellets). It would seem to make a lot of sense to me that this may be the cause of differing results / grouping problems with pellets from various production batches, in paticular when the factory has carried out referbishment work on the tooling creating small changes in pellets.
I have certainly noticed visual differences between pellets from different batches, as I am sure we all have from time to time.
The pellets that I have written about in my original post, do visually have a larger (about 30% bigger) diameter flat area inside the bottom of the skirt than my previous batch. This leads me to think that the weight distribution (C of G) of these pellets is too far back (as in 2 above), I know that the pellet does not have controls, but the same unstable flight would result.
Hope I haven't bored you to much ! heres the web link