De acordo com o HP Wiki
Bombarda is the incantation of a charm used to provoke small explosions; one use for this explosion is to blast open sealed doors or to blow bars off of windows. The incantation Bombarda Maxima is a stronger, more powerful variation of this spell.
EXT. DARK TOWER - TERRACE - NIGHT (SAME TIME)
Wand flashing, Hermione tries spell after spell.
HERMIONE
Dunamis! Liberare! Annihilare!
Emancipare!
No. No. No. No.
SIRIUS BLACK
You might try --
HERMIONE
Quiet! I'm trying to think.
She resumes pacing, MUTTERING furiously. Then... stops.
Turns.
HERMIONE
BOMBARDA!
KA-BOOM! The CELL DOOR ROCKETS to the sky.
SIRIUS BLACK
That'll do.
Considerando que
The Reductor Curse (Reducto) is a curse that can be used to blast solid objects into pieces. It is rather easy to reduce a target to a fine mist or a pile of ashes.
É descrito como tal na primeira vez que é mencionado.
“He had soon mastered the Impediment Jinx, a spell to slow down and obstruct attackers, the Reductor curse, which would enable him to blast solid objects out of his way, and the Four-Point Spell, a useful discovery of Hermione’s which would make his wand point due north, therefore enabling him to check whether he was going in the right direction within the maze.”
Harry Potter and the Goblet of Fire, Chapter 31 (The Third Task)
Então, podemos ver algumas diferenças. Um cria uma explosão direcionada enquanto o outro reduz o alvo em pedaços. Um é um charme, o outro é uma maldição. Um foi encontrado nos livros, o outro existe inteiramente nos filmes.
Meu instinto é que o público geral esperaria que " reducto " tornasse as coisas pequenas, enquanto que " bombarda " soasse como bombardeio , palavra relacionada com explosões.