Since magic in the HP universe has the property of not having to make sense, one could imagine a spell that simply makes guns not work, or that makes all projectiles move slowly, or that causes everyone within the area to miss what they aim at.

The ending battle of Deathly Hallows pretty much treats wands as if they were guns. You could edit the film to replace all the wands with guns and have very few instances where anything looked wrong. So far HP:MoR has made the magic feel more magical than that.