Check out this video tutorial from Viking Tactics on how to engage multiple targets with a rifle using the 1-5 drill.
The 1-5 drill is designed to both reinforce and expand upon the lessons of the 2x2x2 drill.
Like the 2x2x2 drill, we use the 1-5 drill to focus shooters on driving the gun to the center of the target on each shot and shooting as fast as the sights can be aligned.
The 1-5 drill also introduces the idea of shooting until the threat is eliminated. Despite what we might see in the movies, two rounds won't immediately bring an opponent down. It may very well take 3, 4, or 5. Bottom line: you shoot a target until it is no longer a threat. Training too rigorously on "two shots and transition" can ingrain bad instincts into a shooter that just might surface at the worst possible time and place.