Thanks Max, I rather think it is just due to the tightnes of the lead into the rifling from the chamber because when I do manage to extract the cartrige there is very prominant engraving of the rifling right up the length of the projectile and is the same for other FWBs.
I have a 1413 Anschutz and the
FWB seems to shoot better.It sometimes has the same problem but it is much older and has had the chamber renewed by a reputable gunsmith. (the barrel is now about 30mm shorter because of that now)
I was once told that instead of using oil on the trigger assembly, use graphite powder for the same reasons you give above and it would also last better and make it lighter and more consistant.