a) F points right.
Please note the an answer twice as large will be obtained if the currents are considered to flow past the projectile. Actually the current is shunted away from the remaining part of the rail by the projectile; the field is then that at the end of two semi-infinite currents; i.e. the geometry of Problem 18P in Ch. 30, resulting in a field strength of one-half that of infinitely long currents.
Thanks to Janusz for pointing out this subtlety.
b) vF = 2.3 km/s.