Best approximation for what you need would be a shotgun + spear combination.
It seems that what you're creating makes long-range firearms pointless, so let's go short range. Shotgun can take an impressive range of ammunition, and it's so simple design it can be built into a all-metal spear or pike no problem. It can even be worked into detachable-magazine-fed firearm, with magazine being round, of course, to tightly wrap around spear shaft. Reloading mechanism can be triggered by turning the magazine, for example. Or even better: it can actually be a drum, like in Puckle Gun. Unless it will be too unwieldy, it can be a some kind of improvement over Jover and Belton flintlock Flintlock breech-loading musket (which has a tube magazine). This, of course, presupposes "modern" ammunition in the form of manufactured rounds, and not paper musket ammunition, as the latter will make the whole idea very unwieldy, indeed.
Look at this idea as described in Blood Rites, Harry Dresden Chronicles book:
Instead of a second shotgun, though, he drew a weapon made of plain,
nonreflective steel from the van. It was modeled after a boar spear of
the Middle Ages, a shaft about five feet long with a cross-brace
thrusting out on two sides at the base of the spear tip—a foot and a
half of deadly, matte-black blade as wide as my hand at the base, and
tapering down to a fine point at the tip. There was enough mass to the
spear to make me think that he could as easily chop and slash with the
edges of the spearhead as thrust with the tip.
The butt end of the spear ended at some kind of bulbous-looking cap of
metal, maybe just a counterweight. A similar double protrusion bulged
out from the spear shaft at the base of the blade. (...)
“You should go with a shotgun,” Murphy told Kincaid.
Kincaid shook his head. “Can’t shove the shotgun into a charging
vampire or hellhound and hold them off with the cross-brace,” he said.
He settled the spear into his grip and did something to the handle.
The beam of a flashlight clicked on from one side of the bulge at the
base of the spearhead. He tapped the other one with a finger.
“Besides, got incendiary rounds loaded zip-gun style in either end. If
I need them, bang.”
“In the butt end too?” I asked.
He reversed his grip on the spear and showed me the metal casing.
“Pressure trigger on that one,” he said. Kincaid dropped the spear’s
point down and held the haft close to his body, somehow managing to
make the weapon look like a casual and appropriate accessory. “Shove
it hard against the target and boom. Based it on the bang sticks those
National Geographic guys made for diving with sharks.”