Can Ordinary firearms' bullets compare with the Giant Clam water bullets, which can easily pierce through reefs and shatter nearly half a meter thick steel-iron stone trees?
Of course not.
The Ordinary powder weapons currently in service in Terra, with a muzzle velocity around three times the speed of sound and kinetic energy around 4000 joules, have no problem shooting people or Ordinary iron armor, but they are indeed useless against Armor Suit, a clearly bygone epoch technology relic from a past era.
Even the "Alloy Heavy Armor," made bulkier and weighing a couple of hundred pounds due to the outstanding physical qualities of Terra People and material science craftsmanship, can easily block ordinary bullets. Only firearms with an Alchemy Energy Module can pry open these real iron cans, whose armor thickness is measured in centimeters.
In other words, Ordinary firearms can't even break Ian's outer layer of Primordial Sand Armor, let alone wound him.