Well, without any data on the pellets, we can only look at other possibilities- the rifle is either so underpowered it is allowing the pellets to tumble- as the 240 hasn't been made for a few years, where did you get it and are you reasonably confident it is shooting at an appropriate power level (i.e. above 400fps)? Other possibilities are that you have a burr on the barrel crown or some sort of obstruction on the front brake- i.e. hole is too small allowing pellet edge to catch, or a burr or deformation of some sort.