A crossbow for shooting projectiles includes a frame with flexible bow arms and a bowstring connecting the arms, a trigger assembly for retaining the bowstring in a cocked position and releasing to propel the projectile, and a hollow barrel with interior rifling into which projectiles are loaded and through which they are expelled. A firing bolt is connected to the bowstring and located in the barrel to transfer force from the bowstring to the projectile. In the prior art, such firing bolts contact the barrel interior, resulting in friction and wear of the rifling. In this invention, the barrel includes an elongated slot (317) therealong. The firing bolt (324) is slidably supported on rails (329) on the exterior of the barrel. The firing bolt includes a driving member (338, 327) which extends through the slot to contact the projectile, but does not contact the barrel interior walls, thus propelling the projectile without causing friction and wear within the barrel.