This was posted on a recent thread on the RugerForum.net (DA forum), sounded like pretty good advice:
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Ah yes, dry firing ... another one of my favorite subjects. Let's start with the mechanics of the gun. With exception of the bore and the chambers in the cylinder, your revolver doesn't know the difference between dry fire and live fire. The internal parts all get exercised the same except you are much more likely to damage a gun from dry firing.
Dry firing to break a gun in is not wise. Yes, a few parts will smooth up but mostly what you are doing is wearing all the parts prematurely. Here's an analogy that is a bit over dramatic using 60 grit sand paper and assuming the surfaces of the sandpaper represent the mating surfaces of internal parts. Start by placing the sandpaper with the rough sides facing each other. This would represent a mating surface in your gun where both parts are rough. Rub the two pieces of sandpaper together and soon you will see both pieces have worn the roughness off each other. Both pieces will be notably smoother but way thinner, indicating considerable wear. Now place the rough surface of one sheet of sandpaper against the smooth back on another sheet and rub them together. This would simulate a rough part rubbing on a smooth part inside your gun. As you continue to rub them together, the smooth surface now becomes rough and the rough surface stays about the same. For internal parts, that means the smoother mating surface becomes rougher and wears, but not as fast as if both surfaces were rough. Last, place the sheets of sandpaper back-to-back with the smooth surfaces facing each other. You can rub a long time with minimal wear and in fact you will polish the surfaces. This simulates two smooth parts rubbing together. When you dry fire or live fire, the parts behave much the same as the sandpaper. If you start off by smoothing the parts mating surfaces, wear is minimized and the gun will last much longer. If you dry fire to break a gun in, don't expect it to last near as long.
The reason why dry firing causes more damage is twofold. First, people tend to be a bit more aggressive when pulling the trigger in DA or cocking the hammer in SA when they dry fire. This makes the cylinder rotate very fast and come to an abrupt halt when the cylinder latch contacts the cylinder's lock notches. This peens the lock notches and the frame's "window" for the cylinder latch. It doesn't take long and the cylinder lock-up becomes loose. Additionally, the hand pushes a ratchet to rotate the cylinder. These two surfaces wear which retards cylinder timing (carry-up) and the faster you operate the gun, the faster they will wear.
The second issue with dry firing is purely in the numbers. You are more likely to dry fire a thousand times than to live fire a thousand rounds so the revolver is subjected to more repetitions. Usually internal wear has more to do with how you dry than the number of times you dry fire. If you pull the trigger or hammer more slowly, you will minimize peening the cylinder and frame. Lubrication does not prevent peening.
Now for "Snap-Caps". These have to be about the most worthless things ever made and even worse, they give the owner a false sense of security. Snap-Caps do absolutely nothing for the gun's internal parts. Yes, they do cushion the firing pin but they don't even do a good job of that. Ruger's firing pins are made of a copper and steel alloy that are nearly immune to breaking so it is very rare to see one break. Snap-Caps are worth using in the older single and double barrel shotguns where firing pins are more subject to breaking. In modern guns, they do absolutely nothing except to occupy space in the chamber instead of a live round.
Ruger's manuals say it's OK to dry fire but what they really mean is you won't break parts (especially the firing pin) by dry firing. They do not address excessive or improper dry firing that causes wear or damage. Normal dry firing for training or doing a function test on your revolver is not a problem but sitting in front of the TV and clicking away countless times is. When I had my gunsmith shop, it was pretty common for customers to bring in guns with excessive wear. When I asked them how much the gun had been fired, I often got an answer like "I've only fired a couple boxes through it". My next question was about dry firing where I usually got some dumb look instead of an answer.