Firearm comprising a system for moving ammunition; said system comprising: - a movable assembly configured for pushing a piece of ammunition into a breech ring of said firearm and/or closing said breech ring, under control of a motor; - a feeding device configured for feeding said piece of ammunition into a position situated between said movable assembly and said breech ring, and comprising a star loader having a rotatable structure provided with radial arms; and - a synchronization mechanism co-operating, on one side, with said motor and, on the other side, with said feeding mechanism; said synchronization mechanism allowing for selective transmission of motion, under control of said movable assembly, from said motor to said feeding mechanism, when said movable assembly is in a position away from said breech ring; wherein said synchronization mechanism comprises an intermitter comprising: - a driven part rotatably integral with a rotary element of said feeding mechanism; and - a driving part configured to be rotatably actuated by said motor and to intermittently transfer said rotation to said driven part, when said movable assembly is in a position away from said breech ring; wherein said rotary element is implemented substantially as a shaft supporting said radial arms of said star loader; and wherein the axis of rotation (Y-Y) of said shaft is parallel to the axis of rotation (X-X) of said driving part; characterised in that said movable assembly is configured for moving said driven part and said driving part close to each other, selectively engaging said driven part and said driving part when said movable assembly is in a position away from said breech ring. - a movable assembly configured for pushing a piece of ammunition into a breech ring of said firearm and/or closing said breech ring, under control of a motor; - a feeding device configured for feeding said piece of ammunition into a position situated between said movable assembly and said breech ring, and comprising a star loader having a rotatable structure provided with radial arms; and - a synchronization mechanism co-operating, on one side, with said motor and, on the other side, with said feeding mechanism; said synchronization mechanism allowing for selective transmission of motion, under control of said movable assembly, from said motor to said feeding mechanism, when said movable assembly is in a position away from said breech ring; wherein said synchronization mechanism comprises an intermitter comprising: - a driven part rotatably integral with a rotary element of said feeding mechanism; and - a driving part configured to be rotatably actuated by said motor and to intermittently transfer said rotation to said driven part, when said movable assembly is in a position away from said breech ring; - a driven part rotatably integral with a rotary element of said feeding mechanism; and - a driving part configured to be rotatably actuated by said motor and to intermittently transfer said rotation to said driven part, when said movable assembly is in a position away from said breech ring; wherein said rotary element is implemented substantially as a shaft supporting said radial arms of said star loader; and wherein the axis of rotation (Y-Y) of said shaft is parallel to the axis of rotation (X-X) of said driving part; characterised in that said movable assembly is configured for moving said driven part and said driving part close to each other, selectively engaging said driven part and said driving part when said movable assembly is in a position away from said breech ring.