It's both useless and too powerful. With both the ammo *and* the gun being rare, who would ever pick up the ammo if they saw it? It's a waste of inventory space. Same with the gun. Unless the gun and ammo spawned on the same floor, I don't see anyone using it.
On that note, though, it would kill Cybie in an average of four shots. Three if you got even slightly lucky.
It's pretty much the Railgun, but more obtuse.
... Though, it does give me an idea for a unique Sniper Rifle that could work. Fueled by either 9mm or cells, where the normal fire mode is nothing special, but the Special fire Mode would increase accuracy, give a +1 or +2 bonus to sight range, and up the damage -- But would take a few seconds to get into or out of, and you couldn't move while in.