Very interesting replies, from everyone.
In the past I have seen the 'pipping' with domed pellets & learnt that they were used in bell target shooting for this reason as other styles don't produce the same effect. I had also noticed that some domed pellets fired perpendicularly to a flat steel surface have a tendency to almost turn themselves inside out & moderate velocity pellets thus fired often display a central pip on the non impacting surface although the impact surface appears pretty uniformly flat. Not sure what the implications of that are but on reading Solvos excellent terminal ballistics explanation it made me wonder if / what the minimum / maximum velocity of a RN pellet might be if it were to produce a pip on a bell target. Bell target rifles have to be below 6 ft/lb, so does a very low sub 6ft/lb velocity produce the same effect? Conversely is there a maximum velocity beyond which the phenomena ceases to be (way above 6ft/lb?)
I think I need to have a plink at some paint....