What exactly is it that makes it harmful to dry fire certain types of firearms? And which ones are generally inadvisable to dry fire? Just something I've wondered for a while.

Most of the time the manufacturer handbook will say if it's ok or not

most are good with exception of rimfires.

What exactly is it that makes it harmful to dry fire certain types of firearms?

 

Rimfires strike the edge of the cartridge. Without a cartridge in place, this will peen a divit where the cartridge should have been into the breech face. This screws up the seal, the primer strike(less support behind where the pin strikes) and if the firing pin isn't designed to strike steel it may break.

 

Centerfires generally are ok, that said it can stress the firing pin retainer and some guns, such as the CZ-52 with brittle factory firing pins, can break.

 

You need to check the manual on any particular gun you purchase.

that's why you should get one of these.