Hi Rapidrob,depending on how heavily the projectile is rusted you could try using beadblasting to clean them,it is far less damaging than shotblasting,the beads are much softer than shot so cause no damage to the surface of the metal,(steel)a good operator can take away the surface rust without marking the surface of the metal,be warned though that softer metals like alloy,brass and the copper driving band will be have a peened finish after beadblasting,so try to avoid using the beads on these parts of the projectile,something else to remeber is that steel that is beadblasted will form surface rust very quickly(sweat from your finger prints)so a protective coat will probably be needed,
Hope this helps,
Regards,Don,