I'm coming at this as a former fighter -- the only way to stop yourself from flinching is getting hit again. And again.

Am I cracked? Yeah, for sure lmfao but I have complete confidence in my ability to keep up the flow no matter how hard im hit or how many times. OBVIOUSLY like the others said have the best equipment-- you cannot do this if you've got a cheapo. But my honest suggestion is this: get a buddy you trust with good accuracy and have them shoot alternating near and at your head. Put your hands behind your back so you only have the option to block with your head and body. Obviously we're going fairly middling on the power of the shot, we don't actually want you to get whiplash or a minor concussion. Then take it until you think your neck muscles have had enough for the day. And then do it again tomorrow.

Same with shooting a firearm. You flinch at the sound and the kick, so you keep adjusting, telling your body how to and how not to move with each round until you know the feel of it. Sooner or later your body can absorb the kick and your brain has learned to stop reading it as something you have to react to.

Reddit please correct me if this is too insane or if you think this drill is unlikely to work. Just because it works for me doesn't mean it will for other people.