A firearm which is modifiable by the user such that a firearm barrel is arranged replaceable by the user, wherein the firearm comprises a barrel unit, which is detachable by the user and fastenable to a receiver, which barrel unit comprises the barrel and a locking piece, wherein the locking piece has a backward narrowing rear cone, which fits a cone in the receiver and which has a forward narrowing front cone, and which fits a cone in the centering bush, wherein to the centering bush is rotatably connected a clamping bush having an outer thread which fits an inner thread in the receiver, wherein by rotating the clamping bush in the locked direction, the rear end of the locking piece is centerable by means of the rear cone and lockable to the cone in the receiver.