even at 20ft mate they will be no good pigeons feathers act as a shock absorber and they will just bounce off and i they want have enuff punch to kill with a head shot 
  
and as steveS.Yorks said they are that light they will be all over the place and making even hitting them in a kill zone  pot luck ,if i was you id just stick to the 12mm for all game  
its all to do with kinetic energy and 6mm want be packing alot and will start loosing it fast after its left the pouch