I can’t stand it when a dull author that doesn’t understands anything about tech/science writes “genius” characters.
The pistols the MC uses are shown to be 16th-17th century flintlocks.
They use flint to create sparks that ignite the gunpowder.
Cock the spring -> pull the trigger -> flint strikes metal -> sparks ignite gunpowder

Author says that he transmutes elements of air and soil into bullets and gunpowder to fire without reloading.

MC still needs to cock the spring each time before firing, That could be done with a thumb, but the cocking mechanism is shown to be on the wrong side. Regardless, the rate of fire would be abysmal.

Also, the MC doesn’t need gunpowder.
If he can create gunpowder and bullets he should be able to produce stoichiometric hydrogen-oxygen mixture from water around him. That is easier than making gunpowder from nitrogen + oxygen + sulfur + carbon.
And if he can manipulate matter to such extent what stops him from just igniting the H2-O2 mixture? There is no need for flintlock mechanism is he can ignite things directly.