*click*"DON'T DO THAT!"
I've been lectured by at least a dozen people, most of whom I did not know (self crowned range police) and family members, that to pull the trigger of a firearm while its hammer or striker was engaged was to commit sin.
I don't dry fire other people's guns, not without their permission, that would be rude.
When they're my guns, I will do exactly what I wish, thank you! Even if...they're kind of right.
Dry firing, as we all know is to actuate the trigger of a firearm while the chamber is empty and the hammer or striker is engaged.
Simply put, some guns will suffer no consequence from dry firing, and others can be broken, worn, or damaged.
Lets have a look at some examples of which can be, and which cannot, based on their design. Making blanket statements would be easier, but I'll try to be precise and as accurate as I can, and if I am in error, please, PLEASE correct me. There is enough bad information bogging down the internet, without me contributing.....
Can revolvers be safely dry fired? Yes. Can they all? No.
Some revolvers operate with a firing pin that is integral to the hammer, which when driven forward can place a stress different from that when hitting a primer, aside from the potential to shatter the firing pin, dry firing can result in burring, stress and damage to the chamber.
Other revolver designs utilize a "transfer bar" where the hammer does not strike the primer, but translates its energy into the transfer bar which then actuates the pin or striker. As with all things mechanical, it is not indestructible, and many a cowboy shooter (et al) has suffered the woe of chipped transfer bars or worn out springs from practicing their quick draw skills.
Dry firing percussion revolvers can become a costly habit. Nipple breakage is common, and stress to the hammer can cause misfires on caps. It is wise to use nipple stops or improvised blocks when dry firing.
What about modern pistols? Sure! All of them? Nope.
Semi-automatic centerfire pistols are often considered "safe" to dry fire repeatedly. The "floating firing pin" and firing pin blocks (also found in some revolvers) seemingly snuff out the cries of dissent, but this may not be true on the whole.
Composition, plays a role in how well a firing pin survives repeated usage and stress. For example, the production firing pin of CZ-52 (Vz, whatever) is famously prone to breakage. The stress of kinetic energy applied to the shoulders of the brittle pin can cause breakage in no time. Good thing that decocker is super reliable, right*?
Large numbers of dry fires can cause untimely wear to the internals of even the most modern pistols, either on the springs, the hammer, the striker or the fit of the trigger group.
Rifles? Shotguns? Well...
In my home country of the Philippines it was not an uncommon sight to see soldiers or hired guards playing with their "armalites", and by playing, I mean being bored on guard duty and repeatedly dry firing them.
Makes you feel super safe, right? Although the AR-15 design is solid, and it would take a massively OCD 3-Gunner or inordinately bored Filipino security guard to cause any significant wear to the trigger assembly through dry firing, the wear of the hundreds and thousands of actual rounds fired can cause cracks, particularly it seems on titanium firing pins. Dry firing a lower assembly, without the upper assembly secured is asking for trouble, and can potentially damage the other lower components of your beloved black rifle.
Dry firing centerfire rifles, like the Remington 700, is relatively safe...for the firing pin, but constantly practicing getting that perfect trigger pull can cause burring from the bolt slamming closed over and over again, on an empty chamber.
Shotguns can easily suffer firing pin breakage through dry firing, particularly those with exposed hammers, and without free floating pins. Dropping the hammer on an empty barrel of a side-by-side or old coach gun can cause breakage, spring wear, stuck firing pins and of course hammer damage. As you might expect, modern shotguns are of more robust design, and popular models such as the Mossberg 500, and Remington 870 tolerate moderate dry firing.
It is important to note that many rimfire rifles, semiautos and revolvers can suffer damage to their trigger group, pin, and chambers through dry firing. Those equipped with firing pin blocks, or other measures that prevent dry fire damage mitigate this risk. The Ruger MK II/III is an example of a semiautomatic rimfire handgun that tolerates dry firing, (and Ruger is quite proud of that for some reason).
The short of it is...all things fall apart. Some will fail quickly, some will last through years of stress. When put in the perspective that these firearms are machines that tolerate enormous pressures, and perform repetitive, sometimes very complex movements in their action, the fact that we will eventually need to replace something on them becomes more tolerable.
Some folks will squawk and cluck whenever they hear that *click*, and fastidiously use snap caps to prevent any chance of dropping the hammer on an empty chamber. There is a school of thought that says that dry fire is the best way to develop good trigger discipline.
I disagree with both, and maintain that shooting your firearms promote good trigger discipline, and that dry firing is often the only way to function test or decock certain firearms.
As always the adage of "take care of your things, and they will take care of you" proves more succinct than anything else I can think of.
Be careful with your possessions, firearms in particular, treat them well, and you can expect them to tolerate the stresses you place on them in moderation. Making informed decisions is a vital part of being a gun owner, so get informed.
I've often heard that "asking for opinions in a gun shop is asking for trouble", and so if you aren't certain that your firearm can be safely dry fired, do some research on your own or contact your manufacturer.
*I'm totally being sarcastic, be careful. The CZ-52 decocker either doesn't work, or it does work. A working decocker can wear to become unsafe to use, and it is advisable that you consult a qualified gunsmith for advice, or deactivation of the decocker.