A character can shoot when activated, but can also react to shooting by returning fire. Can also react by shooting if the enemy moves more than 3 inches. There is a penalty of -1D for multiple combats i.e. Each prior use of a combat skill in the Turn. So although a character only shoots once during its activation, he/she can fire multiple times during the turn.
Ayup, when someone shoots at you you can shoot back immediately. My favorite part is when both characters go down.

Dear fellow LAFers what changes to the Pulp Alley would you suggest for three musketeers gaming?
We're seeing more folks using Pulp Alley for Musketeers, Pirates, and even French & Indian Wars.
Like Pat suggested, the only thing that might benefit from a little adjusting is Shooting. And I've seen three ideas for this...
1. One Shot Per Turn --- As Pat wrote, simply limit each character to 1 shot per turn. This kind of assume that the character is picking up guns and/or reloading in addition to their normal activation.
2. Action to Reload --- This version also limits characters to one shot per turn, but requires a character to spend an action reloading before they can reload... or spend an action looting a loaded gun.
3. Limit Shoot Dice --- By limiting the Shoot dice to something like 1d for Followers and Allies, and 3d for Sidekicks and Leaders, you're also limiting the number of times they can shoot in a turn -- just in a different way. In addition, this version suggests that higher level characters are more likely to carry more than one firearm and/or pick up other firearms and/or hurl random knives, bottles, and cookware at an enemy.

I suggest trying it a couple different ways and seeing what gives you the right "feel" for your games. And please let us know how it goes!

HAVE FUN