A rifle having a straight pull bolt assembly with a bolt handle that extends outward and downward from a bolt operation mechanism. The bolt handle rotatable a limited rotational range forward and rearwardly about a horizontal axis for operating the mechanism. The handle can be reconfigured to position its rotational range about the horizontal axis and can be attached to the mechanism at either left or right sides to provide for ambidextrous operation. Bolt heads, lower frame members with magazine wells, and barrel assemblies can be readily swapped to change ammunition. The bolt assembly can be unlocked by firing or by a thumb push button release at the backside of the mechanism. A bolt removal release mechanism is forward of the thumb pushbutton and an ejector includes a spring bias and cam follower surface to reliably eject casings.