There are a few reasons why a shooter might want to practice loading only one or two loose rounds in a cylinder.
1 – Range practice with a few empty charge holes are the revolver equivalent of ball-and-dummy drills. Can this be done with dedicated dummy rounds? Yes, but leaving a few charge holes empty saves the work of collecting the dummy rounds at the end of the shooting session. Also, your local indoor range may not want to shut down the range so you can recover your dummy rounds while other shooters are waiting.
2 – Few things will help you develop a “flinch free” shooting habit than constantly catching yourself anticipating the revolver’s recoil only to then have the hammer fall on an empty cylinder.
3 – A cylinder only partially loaded teaches a shooter to “shoot through” the cylinder rather than A) expect the rounds to automatically be indexed or B) demonstrate how quickly you can cycle through to the live rounds rather than trying to manually index the next round.
Once when I was at the SIG Academy I set up the following drill: The “good guy” had shot down two of three attackers who were trying to assault (kill) his family. The good guy had fired off all five of his cylinder’s rounds. He was now attempting to reload. As it was an artificial drill I told the shooter that he would have time to load two rounds (all I gave him to reload) but that he was to reload under the clock. As soon as he loaded histwo rounds he was to return to the target and get off two accurate shots. The shooter rushing to get the rounds in loaded both rounds, turned to the target and (Still racing the clock) pulled the trigger three times. “Click!-Click!-Click!” Believing the rounds had misfired he dropped his shoulders in defeat and we suspended the drill. “What!?!” I screamed “are you doing?” The rounds “failed me” he told me. “No!” I continued, “the gun is still hot. You’re so used to loading the gun and indexing the cylinder that you think the gun will do it for you in an emergency! Now get back on target and fight!”
4 – A partially loaded cylinder can break a shooter of the deadly habit of believing that the gun is only partially loaded. Here is a simple math test. If the cylinder holds five rounds and you load the cylinder with four rounds, by what percentage is the gun loaded? (Here is a “hint,” four divided by five is eighty percent) Do you know the answer? Did you say eighty percent? Well then move yourself to the back of the class. Four rounds loaded into a five round cylinder means the gun is loaded one-hundred percent four times over. A single round loads that gun one-hundred percent. At least one of the four policemen killed during the Newhall shootout was reported to have died with an open revolver loaded with five rounds. Might he have been better served by his instructors if they had taught him to partially load the cylinder and then return to the fight with a fully loaded gun (Five rounds meant the gun was loaded one-hundred percent, five times over!) rather than leaving him to die trying to fully load the cylinder?
That said, the following three drills are offered only as an exercise. I would rather a 2nd snub, a speed loader, or a Speed- or QuickStrip be used in lieu of deliberately planning on reloading with loose rounds, but sometimes uncommon scenarios do occur. By way of example, sometimes rounds come loose from speed loaders, Speed- or QuickStrips. Remember that during the infamous FBI shootout at least one agent tried to reload his revolver with the loose rounds from a box of fifty kept in his car’s glove box.