Firearms are an addition new to Kingdom Hearts V. They are a new way to combat enemies. While not as efficient as a Keyblade, firearms are useful in many situations a Keyblade just isn't. For example, a firearm might be better in a boss battle in a larger area than your Keyblade because you can stay farther off and still cause damage. Firearms cannot deal the final blow, however. That much must be done with a Keyblade.
There are several types of firearm, but the most common is Pistol-type. Every character has a set of firearms specific to themselves. The only choice you're given is with Harley. With his Primary, you have the option of choosing the HRO-R hybrid assault/sniper rifle or a pair of submachine guns.
Firearms have a limited clip size, but an unlimited amount of ammunition. When weapons are reloading, the character is rendered vunerable until the reload is complete. Normally an audio cue like "Cover me! I'm reloading!" or "Damn! Out of ammo, watch my back while I reload!" will alert you to the noise.
All weapons have a specific fire mode depending on the gun, like Bolt-Action for Coltrane's Sniper Rifle (slow fire rate), to Semi-Automatic for Shorty, Harley's shotgun(no need to wait) or Full-Auto for the Speedster (rapid-fire).