Normally, it's taking aim that takes all that time. To bring that to the game, you may increase the number of attacks possible in one round, but make "locking on" to new targets a standard action. I believe Jagged Alliance 2 had a similar combat system.

Also, you can make burst fires that are similar to cone-shaped spells. For example, every bullet you would additionally fire during that burst widens the cone, but lowers it's DC (or something similar).