Guns discussions that want balance? Seems like a misplaced expectation
Literally one of the most famous idiomatic phrases in America "You don't bring a knife (or in this case sword, axe, glaive-guisarme, whatever) to a gun fight"
It took about 300 years for the gun to obsolete melee weapons, and another 150 years for military tactics to catch up to that fact.
Early guns functionally were encounter powers as the OP suggests, that would take a minute or more to load and which were extremely unreliable except in perfect weather conditions. As late as the early 18th century trained longbowmen were competitive with musketeers, and primary advantage of the firearm was just how easy it was to muster conscripts with guns compared to conscripted longbowmen in any useful numbers.
Unlike modern smokeless powder, true gunpowder is not a high explosive and as such musket balls had relatively low velocity and had to rely primarily on mass for killing power. Killing a large game animal with a black powder musket wasn't easy, and absolutely massive guns that were practically handheld cannons were required to do it, and even then not reliably and even then only late in the development of blackpowder weapons. Literally, you could hit a charging elephant with a 4 bore and just stun it - quarter pound slug bouncing off its brain case if you didn't place your shot well or wait until it was close enough. If you were trying to kill a rhino or a hippo or a bear, you were probably as well off with a spear as a musket provided you had the proper training.
It's perfectly possible in a 15th or 16th or even 17th century inspired game to balance firearms with older weapons and still have verisimilitude. The trouble is that this balance is an awkward one for D&D because the balancing factor remains that firearms are a lot easier to be effective with if you don't have a ton of military training than a sword, spear, glaive-guisarme or whatever. The balancing point in D&D terms is level. Below X level you are better off with a firearm, and above X level you are better off primarily focusing on something else (possibly with a brace of single shot pistols as supplemental weapons). Encounter powers aren't fully realistic but they do capture the spirit of an early blackpowder weapon. And that creates a problem, because NPCs get more out of gaining access to a firearm encounter power than PCs do given their array of powerful options already. Like who really gets more advantage out of wheellock musket or (
shudder) a 4 bore elephant gun - a typical adventuring PC or a 1st level NPC. Sure, a PC would like to have a one-shot weapon doing say 1d10+2 damage or (
shudder) 9d6 damage, but would they like it if hobgoblins had 20 such weapons? It's a problem because low level D&D becomes more lethal and stays lethal for a longer period. Numbers of combatants start to matter more than the skill of combatants.
Move up to 19th century or 20th century firearms, and its an entirely different game and yeah your magic sword starts to feel a bit puny brought to a gunfight. Now your wizard is outclassed at low levels by every well-armed thing and a platoon of smart low-level characters can make life just hell for high level characters. Napoleonic hobgoblins wheeling out anti-personnel artillery introduce though the real problem with advanced technology in a heroic setting - the gun isn't the limit of destructive power.
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