In the Dune 2021 we see Duncan Idaho being shot at with a projectile weapon. He had to use his sword to pry the projectile off his shield. But imagine a dozen of those projectiles working their way through the shield at the same time. Even Duncan couldn't survive that. So why don't armies use those projectile weapons on a massive scale? Where are the rifles and gatling guns that fire these projectiles? Why isn't every soldier equipped with one?
Briefly: The projectiles move too fast and therefore they would typically bounce off the shield. As Mithoron mentions Herbert was using his story to bring give us a space fantasy. And projectiles in Herbert’s universe are obsolete because of velocity issues with regard to the shield-armor. Think about how the hunter-seeker going after Paul must move slowly to pierce the shield.
This is input from a person who is well versed on the books and watched the new movie once. And is blind so experienced the movie via audio description service.