The gun discharges with a sudden and deafening boom, filling the air before you with thick gray smoke and causing the reinforced metal railing to groan ominously.
The slug fired, a round-nosed lead cylinder a little more than an inch across, with a slight left-hand spin and a hollow, slightly flared back end, weighs about four ounces and is currently traveling at one hundred and seventy-three-point-five-two percent of the speed of sound, leaving a sharp snapping noise in the air behind it in the short time it takes to travel the two-point-three miles from the muzzle of the gun to the torso of an unlucky mercenary, who is suddenly struck backwards as if by a massive hammer, a single, perfectly round hole in the front of his armor, and a considerable cone of gore, shattered bone, and tiny, still-incredibly-high-velocity fragments of lead erupting from his back.
A single, particularly large specimen of such fragments flies another ten feet to embed itself in the cheek of a hostile caster. Perhaps next time you'll get luckier, that one was right on target until it got out of your influential range. Now to reload.
It is at this point that the first of the advancing undead encounter the fortress' minefield, beginning some five hundred meters from the walls in all directions. A small, loosely-spread group of five or six zombies dully notice an unusual humming sound in the fractions of a second before the earth beneath their feet erupts in fire and a storm of shrapnel tears everything within fifteen feet into bloody shreds. Five kilograms of dynamite throwing a large sack of scrap metal into its general vicinity tends to be rather destructive.