To preface this post, I saw a friend’s Snapchat of himself and some others out in the woods shooting targets with various firearms. One of his friends was clearly left handed and was struggling with shooting and cycling what looked like a Mosin Nagant. It got me thinking, were there enough left handed soldiers to have made countries have some of their issued rifles changed to accommodate them? Or where they just left to figure it out? Or were they just put into other duties? I was mostly thinking of the period where bolt action rifles were the main weapon for front line soldiers. But any interesting info is welcome! Thanks for your answers!
The short answer is that they just made do and learned how to operate their weapons as best they could with their off hand. I suppose that once open order drill became common that some commanders may have allowed their left handed soldiers to fire left handed, but to my knowledge there are no drill manuals from any country that cover left hand shooting, meaning that everyone was expected to shoot right handed.
I don't have a good answer for you as to why military longarms are right handed, save that left handers are a significant minority of the population, and as firearms developed their design utilized a side mounted lock plate for the firing mechanism. Standardizing the side the lock was mounted on was needed due to the close order formations soldiers initially fought in, and the amount of sparks and flash produced by their weapons.
There are a handful of what are essentially in-line muzzleloaders that could be fired pretty comfortably left handed, but none of them were designed as ambidextrous weapons. The Oldenburg Model 1843/48 rifle and rifle-musket has a cheek rest on the left side of the stock for right hand shooting, and the Confederate Read and Watson rifles have slightly offset rear sight notches which make them more suitable for right hand shooting as well.