Fear not the big booms

tl;dr: no need to be afraid of early-modern fire-arms in your campaign

longstanding question about guns in D&D - my experiences no need to worry too much. I have had gunpowder crop up in two campaigns so far - Spelljammer Light of Xaryxis and Hexcrawl25 Realm of the Reluctant King - and in neither did it wreck the campaign.

First and most simply - one of the party was a musket armed Giff in the Spelljammer game and they played the part of a sniper par excellence - but no moreso than the long-bow wielding elf I adventured alongside in Out of the Abyss. Campaign took place in wildspace/on planets and in any case I was playing the campaign as written so no phlogiston nor the ignition risk of gunpowder associated with that. All in all, it was wholly a flavour thing beyond one trap made out of gunpowder barrels.

The players even made fairly large scale anti-shipping contraptions they launched with some spiky trap things included but never tried to acquire or incorporate gunpowder for whatever reasons.

In the second campaign - the Hexcrawl25 game - gunpowder had a lot more of a place but still did not break the game or ruin the flavour. It was introduced somewhat as an accident because I had resolved to try and use as much of Knock#1 as possible in that campaign and so when they searched the back rooms of a tavern unexpectedly I decided the '20 Gunpowders' table could be fun and it went from there.

The first thing they found was a heap of 'dragonpowder' - magical thermite essentially that "a wizard left there" which they stuffed into a bag and scarpered with. Afterwards I used a usage dice system to track depletion for their rather random use of it. It came in very handy for venturing through an insectfolk labyrinth where they used it to burn open some vaults.

With the knowledge that in-world there were such things, they took themselves to a goblin-market and traded various battlefield lootings for some more explosive powders - a handful of vials of saintsbone and a pouch of organic goblins-own gunpowder that could theoretically be self-replenishing. After this they took themselves off to commission a shotgun/blunderbus type weapon for the dwarf and after that considered themselves ready. These were a few mini-quests over a series of sessions as they were traipsing too and fro across the hexmap so I considered those things well earned.

Their final set up was for the dwarf to have the shotgun with their small squire riding on their shoulder and serving as a loader in fights. They also put a saddle-horn type mount for the gun on the top of their big lizard beast of burden so that the two small minions could operate it from beast-back while everyone else dismounted to fight on foot.

In the end, it never came into play as a game-breaking thing - it let the dwarf fighter participate in fights at range alongside the sorcerer and artificer - at about the same level of effectivenss which I could live with. Often at close quarters the noise of using the shotgun stopped it from being a 'first strike' weapon and the party as a whole leaned more on magic.

The funny thing is that these gunpowders came into play at the climax of each of the dungeons in Knock#1:
- In Citadel of Evil a batch of dragonpowder bundled to a vial of alchemists fire was catapulted into the main stairwell and set off an inferno that both torched out the top floors of the castle but also cut off the main villains line of retreat.
- In Praise the Fallen the main villain was about to teleport away on their initiative when the herbalist-themed artificer catapulted a bundle of all the saintsbone at their feet for great fistfuls of damage dice. This also threw some damage on most of the party and deafened them but it did succeed in over-killing the villain before they could flee
- In Zaratazarat's Manse when they found the mysterious monster-spawning slab they dumped most of their remaining dragonpowder onto it to burn through the slab while they fought the rest of the things around them. If I recall correctly some poor unfortunate bullywug or locathah spawned onto the table in the middle of that inferno and did not have a fun time either.

Otherwise, the dragonpowder saw the most use for burning through things and general destruction and mayhem against non-moving foes or other obstructions. Weirdly, they also decided during one significant night carousing to try inhaling dragonpowder and some more rolls on the goblin activity table established it to be a fairly effective hang-over cure.

There was two other notable uses of explosives throughout the campaign - first to cover a prison escape and second to collapse a doorway with hostiles on the far side.

The prison escape hinged off roping in some dwarven artificers who were part of the group being rescued to set up the trigger mechanisms to turn the explosives the players could supply into the time-delayed inferno they needed. I was pretty happy with this, as it was a pretty cunning plan overall relying on a pushing a rigged mine-cart onto a lift to the enemy camp above and then running like hell. The challenge for the players was not trivialised by the availability of explosives, so all to the good.

The door collapse was during the final session - while fighting thralls in "The Aboleths Dream" by Elven Tower the party had wedged the door leading into room seven while they fought other thralls. More thralls gathered from room(s) eight to bash down the door and had splintered it open Shining style when one of the party Catapults a bundle of vials of explosives wrapped around an alchemists fire. Big kaboom, bought them time to run off, and on their exit up stairs one they touched off the last of their gunpowder to shatter the entrance and let them get away.

In two separate campaigns the players never really went all-in on the potential of having access to explosives - except for the above 'big boss killer' moments. We were not seeing land-mine equivalents, doors being blown off hinges, explosives rolled into enemy strong points or similar item-focussed problem solving.

To mirror this, I did not lean heavily into explosives being in the hands of adversaries - the RAW magical ranged attacks of Astral Elves in the Light of Xaryxis campaign were sufficient for them. For the hexcrawl, the two major groups of adversaries so far have been mobs of cultists ambushed in tunnels and great beasts fought out in the open, neither of which have been made boring by the dwarf having a shotgun. The tunnel fights had a big element of sneaking about to not draw down the rest of the dungeon which kept things interesting. The big beasts were generally big enough that even a shotgun was not going to cut much mustard.

The main lesson I took from all this was that the gamebreaking potential of low-rate of fire weapons is overstated - the magical fire-power of standard classes of characters is equivalent, if not better. I cannot promise if you throw in some odd explosives for flavour, your players won't wreck your campaign but two completely different sets of players (different ages, nationalities, game formats) did not leverage them to wreck mine so maybe you'll be fine too.