My flinch is worst when things go slow. So, first order of business is to make every effort that will prevent slow fire or no spark, ie check flint, wipe pan and frizzen, clear touchhole, bank prime against barrel, etc., more of a routine that helps me feel in control. Never hurts... Next, I look for or at least anticipate pan flash and fireflower at muzzle, as signs that things are going well not as ugly surprises. I know this runs counter to most, but I can't NOT think of something as James put it...
Also, I understand the desire to do offhand, but I noticed my offhand flinch is less or occasionally gone after shooting flint chunk rifle. Removing the instability of offhand I think allows focus and a string of good shots delivered to the target inspires faith that the flintlock is capable of shooting just fine. I believe at least for me that flinching is the result of lost confidence and frustration, so anything that minimizes those will help.
Take this as observations only, as I struggle with flinching constantly and only am making some progress one I started systematically eliminating factors and coming up with strategies that work, if only because they give me a routine to follow and some hope
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