The outdoor “Cannon Walk” at Fort Sill, Oklahoma, is anchored at its west end by “Atomic Annie,” one of America’s arsenal of forward-thinking guns that fired nuclear warheads. Now Fort Sill has become the place to see two of the America’s most inventive weapons, because just down the street from Annie is the new U.S. Army Field Artillery Museum and its star exhibit (at least to us), the Mule Gun.