I mean it legitimately is a magical gun, which for reference are rather weird.
From overlord volume 13
Page 240: It used mana to fire bullet-like bolts and it felt like a crossbow. Shizu had said something about the gunpowder not showing any signs of combustion or something, but Neia had not understood any of it, so the explanation was wasted on her
It produces a pew sound when not on full burst page 253: she could hear Shizu’s spell gun going pew pew pew
It has a full burst mode page 253:“…Reloading… full burst.” There was a piercing kyuuuuun and then a cacophonous dakka dakka dakka dakka thundered down the passageway. Unlike the pew pew sound from before, this was a sound that was filled with an oppressive brutality
Despite not showing signs of combustion, it does seem to produce fire when shot
page 274: The muzzle spat fire, and the great demon grabbed its shoulder and stumbled back
And from overlord volume 14 page 259: Compared to Shizu’s assault rifle type Arcane Rifle, this enemy’s Arcane Rifle was more of a heavy machine gun type according to Cocytus
So following this the gun uses mana to launch projectiles but also has gunpowder that doesn't combust like real gunpowder despite producing flames when used in the gun.
TBF to new world, even if everyone did march on nazarick by word of god they simply couldn't take the tomb, ainz and coe are so out of scale for the setting they legitimately wouldn't lose even if the whole world turned against them