Were ever the multi-barreled gunpowder weapons effective?
I feel like you meant to ask that to my blog and not @its-spelled-maille but I guess I’ll answer it nonetheless.
Never as effective as single-barrel ones, no. The Nock gun was massively overpowered, pepperboxes aren’t as good as revolvers, duck foot pistols are kinda stupid… generally they’re either novelties or technological dead-ends, easily surpassed by better thought-out designs.
PS: although it does make some kind of sense in the middle ages when that was the only way to get repeat fire, but at the time firearms weren’t that effective period. But we can excuse everything to ribauldequins because a) they look amazing b) their name means bitch in old French.
PPS: Okay so if you’re looking for slightly more modern designs there are indeed two types of multi barreled guns that are useful.
Hunting shotguns, what with the twin barrels and such, had an unprecedented fire rate until semi-automatic stuff became available.
Gatling-type rotary cannons, because if you try to fit such a high volume of fire through a single barrel it would melt and the internals would explode.