If I decide to purchase any new long arm muzzleloading rifles, they will have to be ambidextrous. Doctor doesn't really want me shooting from the shoulders do to progressive osteoarthritis, but if I decide to do so, then I will have to distribute the recoil load equally between both shoulders.
That pretty much means a custom rifle with the L.O.P. much shorter than most current factory firearms are offered at. Nothing longer than 13" so that I can shoot properly if wearing heavy layers of winter clothing.
Fortunately, I started shooting from the left side in my teens as a challenge from some of my peers who dared me to do so. When I proved to be the only one who could accurately shoot left-handed that day, I continued to practice as I got older. Even with my flintlocks. I never had any issues shooting a flintlock because I never took flinching from the flash in the pan into account when I pulled the trigger all by myself at age 17 for the very first time. So, shooting a right-hand rifle, left-handed, where the flash is even closer to one's face, never bothered me in the least.
Other's mileage may vary.