There are lots of methods people use to help them defeat the reflex to flinch. My method was so effective that now I don't even blink my eyes when I shoot my Glock 21.
It's not complicated. All it takes is time and money.
Just fire 500 rounds of .44 magnum.
I have a Smith & Wesson Model 629-2 Classic Hunter.
The violence conveyed on the user end of this gun is dramatic to say the least. Impressive may be a better word. The recoil combined with the fireball and the general concussion of the thing usually, literally, shakes the dust from the rafters.
When I put 50 rounds through this little beast and then shoot my .45acp, it feels like a .22lr.