No, you would not be legal.Theoretically. You take a .410 slug gun; handload slugs with an additional single small piece of lead between the projectile and the powder, you are now not shooting a single projectile but a .41 cal round ef' you and then the small piece of horse you rode in on. Easy to suppress and safe for thermal optic mount. You would be legal right?
ETA:
why not a 12 or 20 slug