I'm afraid this post isn't going to be very good. It's difficult to relate rifling to D&D, for while the development had great consequences upon the future of warfare after the technology was implemented en masse, I'm afraid the lack of the technology wasn't something that people went around noticing or writing psychological treatises about. It would have been nice to have something written by Kepler or Torricelli about what a darn pain it was - a socially relevant pain, that is - that the guns on the battlefield didn't fire straight, but alas if it's out there it's in either German or Italian, and no one has quoted it yet in the shot of my ears.
So the lack of rifling and its influence on D&D is somewhat obvious. There was no rifling and D&D more or less exists without guns that fire straight.
We can't call it quits there, unfortunately. This is the sort of problem that's going to come up again and again as the technologies become more and more advanced. We're dedicated towards writing them right through to the end, so we'll just have to get proficient and digging out the pertinent details and do the best we can.
It must be said that, more or less, D&D exists without guns. Most persons are happy to run campaigns that take place in a sort of ethereal historical situation, in which there are no guns, or various other chemical technologies ... though of course there's experimentation and social development and extremely heavy armor of the kind that wasn't developed until the 15th century: a sort of potpourri of selected technological developments. There's nothing wrong with this, of course. My pedantic nature feels the need to point out that this potpourri is mostly the result of poor scholarship and a lot of misunderstandings about historical developments of weapons and social interaction, but I'm not saying you can't consider the ignorance a blessing and just enjoy the fact that so many disparate elements of human history are tumbled together into the D&D fantasy system.
I'm not demonizing the ignorance. I'm just saying, it's there.
Willy nilly, I think we all keep and toss out the elements of things that most suit our peculiar natures. The D&D world, by necessity, has in it a thousand discontinuities even before you get to the historical dissonance - what with magic, monsters, actual gods and so on, we're not talking about any sort of realistic portrayal of anything. I mean shit, how do I argue my 17th century world is anything like Earth when more than half of Russia is occupied by goblins, orcs and hobgoblins, with an existing Greek and Hindu pantheon of gods fighting it out on planes of existence drawn completely from my own imagination and mixed in with the endless hodgepodge of mythological beasts and places? I can't. D&D is not real. It's important - like any dramatic invention - that it FEELS real, but we're obviously not saying that it is.
So when the question of rifling comes up, and any of a number of other alien-to-D&D technologies, like plastics and satellites and fusion, we're certainly NOT saying you can't toss them into the pot and stir them around to suit your needs. Hell, who says there can't be a race of intelligent warthogs with opposable thumbs from the Plane of Excrete whose attempt to enslave the prime material plane for their shit production isn't aided by muskets with rifling? Could be. What is the party to do, as they face these dangerous hogs with swords and magic? The best they can, that's what.
Thus the onus is upon me to talk about the difference between firearms without rifling and firearms with, and the possible social consequences that could be installed in your world to make things more interesting.
What I have along those lines is this:
Prior to the development of rifling, there was only a certain amount you could do to ensure you were killing your enemy with gunfire. You lined your men up, you got as close to the enemy as you dared (30 yards or so, which is still a great deal farther away than you needed for swords), kept your men as calm as you could and got them to fire as often as possible. It was understood by some commanders early on that having all your men fire at the same time produced a terrific shock effect upon the enemy, which tended to scare the enemy away, thus winning the battle without having to keep firing endlessly into each other without results ... which did happen.
The ball in a musket, even in skilled hands, would bounce against the interior of the barrel and could come out in virtually any direction - which meant that your line had to be pretty straight if you didn't want to kill your own men in the back at close range. And because the time it took to reload could be as much as 30 or 40 seconds, it was possible for the enemy to simply drop their guns (this is before bayonets) and rush at you, particularly if the ground was level. You will remember from high school that you used to run the hundred yard dash in about 14 to 16 seconds (or a bit less), so you know that it doesn't take an eon to cover 30 yards, even if that distance is covered with thick vegetation and the occasional fallen body.
Thus it was necessary to have men whose purpose was to stand with pikes and keep you at bay while the musketeers loaded as fast as they could. This was aided by the flintlock musket, which increased the distance between armies to slightly less than a hundred yards - giving you MORE time to load, which was much more important than aiming. Armies had learned by the 1590s, when the flintlock came into general usage, that you needed your men to form ranks, where the men at the back were loading like crazy and the front men firing ... then running to the rear. This enabled a murderous rate of fire that did much to end the rush-across-the-battlefield tactic - though generals refused to believe that for the next 350 years.
By the time of Gustavus Adolphus of Sweden, Dutch gunners had broken down the principles of loading a musket into 45 separate actions, which if followed perfectly and in the right order could enable a trained man to load his gun up to 40% faster than untrained men. This was the principle reason Gustavus ripped through Germany in the Thirty Years War - more men firing, less time loading, meant he had more effective men on the combat field than his enemy at any one time, since less of them were standing around doing nothing.
Still, without rifling, the weapons themselves were tremendously unreliable. More shots meant greater effectiveness, but aiming was not something you could rely upon. Greater 'skill' with a weapon meant you could load it quickly. It did not mean you could necessarily hit the enemy you were aiming at. Of course, if you loaded faster, you got more chances to hit him.
Rifling made the difference. By causing the bullet to spin as it left the barrel, you gave the missile stability, and stability meant a well trained could hit with his weapon as well as load. And this produced a marked change in how you trained your soldiers.
In the 17th century, you trained them to load quickly and produce as much fire as possible. They had to be calm so they could concentrate on loading ... if you misloaded a weapon, it would blow up in your face and give you trouble. Alternately, if you got the steps in the wrong order, you clumsily had to go back and get them right, which meant fouling up the whole rank standing behind you, as they waited for you to get your shit together.
By the 19th century, the calmness still needed to be there - but not because it took a long time to load. Breechloading had come into being, and you could ready your weapon in very little time. However, standing on the battlefield and hitting your opponent well and true could mean forcing yourself to be calm and aim while bullets whizzed around you.
In any mass battle - the American Civil War, for instance - most of the combatants were largely untrained, and unable to take full advantage of the remarkable weapon the rifle had become. But a well-seasoned, trained troop of men who could stand calmly facing you without firing while they aimed - while you blasted the air as fast as you could - would cut you down like wheat. Again and again the elite British troops proved this tactic.
This calmness had a peculiar effect upon the society, perpetrated by men who had experienced war fought in this fashion. They did not 'bat an eye' at many a hard question or need that might arise. They did not view hesitation as a virtue. They did not view the habit of questioning every decision, or pining for lost opportunities, or self-doubt, as virtues. These are not virtues on the battlefield. They were not seen as virtues in ordinary life.
If you wonder why or how men could be as callous as they were in bygone eras, consider that this was a callousness that was acquired during explosions, with friends and enemies torn apart by bullets, in holocausts of fire and blood.
Compare this, if you will, to what it must have been like to hold a sword, and not a pistol, and stand three feet from a fellow with an axe, and nowhere to run between you. Consider how hard this must have made these men who survived these ordeals ... and how matter-of-factly they took the matter of burning witches in towns, or incarcerating villains, or mass executions of blasphemers. Apply the calm exterior of the British rifleman with the calm exterior of the Viking raider, and ask yourself why they should for a moment feel anything for the woman and her child who are split open by the held axe. And then ask, if you should have lived in this time, according to these principles, amid others who had adopted the same behavior as yourself, how hard you would bring the battle against warthogs who wanted your shit.
Pretty hard, I think.