This is true; there is a change in the barrel's attitude (up and down angle, not whether it's mad or not) following ignition. This is simple physics, really - equal and opposite reaction forces. The moment the projectile starts to travel down the bore, there is an opposite force given to the rifle, however mitigated by the weight of the rifle and the resistance offered by the shooter's shoulder, etc. I am quite sure this is even more pronounced the larger the bullet (read heavier) and the slower the velocity of said bullet (more actual dwell time in the barrel which is recoiling). Add to this rifle construction in terms of sheer mass and allocation of that mass - I'm sure this is why offhand target shooters preferred a muzzle-heavy rifle, beyond just holding it steady. The longer barrel with lots of weight out front makes for a very difficult lever to lift, which should bring the recoil impulse's levering effects on the barrel into tolerable levels. This even more brings into focus the target shooter's admonition to keep absolute consistency with shooting technique and hold. What all this still doesn't answer is why the effects of this recoil-induced barrel rise becoming high POI vs. POA is different for us in the modern age compared to our long-dead brethren. If this force was present then, it should be present now, and vice-versa. If I was continually hitting high, I would change the sights, regardless of the reasons for the higher bullet impacts (assuming both consistency and accuracy). Why not then? I do understand that it makes sense to aim lower, because your sight is not obscuring your target like it would if you had to aim higher (Kentucky elevation???), but I still see some issues. If I was facing a hostile combatant, I would not want to concentrate on their belt buckle, I would want to keep their eyes in view. I believe (my theory) that this is indeed the volley-fire/long-range idea. It seems with the rainbow-trajectories of Sniders, Martini-Henrys, etc., a 200 yard zero would result in a close-range shot over the bow, if you know what I mean. I expect that they would have considered a close-range enemy to be a subject for a bayonet instead. Old ways of thinking, indeed.