I doubt gunslingers had much access to sandpaper
I imagine a real "slinger's" hands were quite callused, fingertips included.
People cracking safes used to sand their fingerprints down so as not to be so detectable...(movie lore)....and as seen in movies, to make their fingertips more sensitive......more sensitive to pain perhaps but not to the tiny tumblings occuring behind a steel dial.
Sleeping with your hand in a box of sand would not "de-sensitize" one's fingertips, most likely the opposite. Might be a mental placebo though.
There are a couple of ways to "toughen" one's fingertips. In any case, while toughening up the fingertips may be beneficial to someone wishing to run their fingers through boards, I can hardly see why it would be beneficial to anyone needing a more sensitive touch..ie; gunslingers & safecrackers.
I have a 50 gallon drum of Lightning grease (great for arthritis)....but i'm hoarding that along with my 5 gallon spray can of DDT and my well hidden Lawn darts!