Getting back to Iron Palm practice.
I have always been lead to understand that there is the -indirect- and -direct- methods.
each of these of course will have sub sections.
But in short, indirect is starting with the mung bean bag, graduating to river stone and finally to Iron shot. Starting with a falling hand technique and culminating with actual striking of the bag from shifting horse and bow&arrow stances.
Followed up with dit da jow after -x- amounts of strikes per hand and per hand form. fwiw, there are many types of jow from light liniments to heavy herbals that break up blood stasis via increased blood flow and pain killing. This method when done correctly will not cause damage, loss of sensation or even callousing.
The -direct- method is where a bin or large urn is filled with mung beans and spear hands are thrust into the beans, then clutched and squeezed. When the beans have become dust, they are replaced with riverstone and the process is continued until the rivers stones are broken. The stones are when the urn is heated from beneath, the muing beans in the urn are not heated. The drying of the stones from the heat will cause them to break down a bit as well.
Once the stone sessions are done, the steel shot or iron shot is filling the urn now and the process continues. the shot is heated and this process is carried on for -x- amount of time until ready for the final mixture which is iron filings. Heated from below, the filings are sharp and the hands can get a little bit cut if the previous process has not been carried out correctly. THis indirect method is the method which often leads to callouses, loss of sensation and other negative effects. But the effect is quicker to achieve than the indirect method. The indirect method can be used to maintain and sustain the Iron Palm. There are other methods to sustain and maintain the Iron Palm as well such as water palm exercises, mook jong exercises and various other methods to essentially maintain that which you have developed through the practice of the method.
I myself prefer and use the indirect method and occaisionally test it for maintenance reasons with some simple breaking tests.
Can’t say that I have ever killed a horse though 
I wouldn’t want to.
A small word of advice for those who would try this method of training.
-BE CAREFUL AROUND CHILDREN AND FRAIL PEOPLE WHEN YOU TOUCH THEM-
Also be careful in how you shake hands with people. You may not realize just how much hand strength you have and you can indeed hurt somebody with no intention of doing so. The slow building of the process is what makes you a little unaware of how much you have progressed.
Be aware as well that these methods will take the better part of a year to get to the beginning of the intermediate levels.
cheers