The M-28 or M-29 Davy Crockett Weapon System(s) was a tactical nuclear recoilless gun (smoothbore) for firing the M-388 nuclear projectile that was deployed by the United States during the Cold War.
http://en.wikipedia.org/wiki/Davy_Crockett_(nuclear_device)
Hrmm ... stats 2-4km range, but:
Both recoilless guns proved to have poor accuracy in testing, so the shell's greatest effect would have been its extreme radiation hazard.
The M-388 would produce an almost instantly lethal radiation dosage (in excess of 10,000 rem) within 500 feet (150 m), and a probably fatal dose (around 600 rem) within a quarter mile (400 m).
Ouch.
Never, ever, tell your players about this.