I remember trying out H&N Hollwpoints (I think they were initially marketed as Beeman) many years ago, in anticipation of Dirty Harry style kills on rabbits (well I was young!).

When I tested them, the only time they "mushroomed", ie. the front cavity opening and folding backwards, was when fired into very dense material like plasticine or putty. When recovered from shot quarry, they showed no more noticeable deformation than a round or flat head pellet. This was at 12 ft-lb levels; they may well perform better at higher velocities.