A firearm with barrel extension comprising: a longitudinal axis; a receiver; a barrel supported by the receiver and including a front muzzle end, a rear breech end defining a chamber for holding an ammunition shell, and an axial bore extending between the ends; a bolt supported by the receiver for axial forward and rearward movement, the bolt comprising a bolt head including a plurality of bolt lugs (56a-d) extending radially outward from the bolt and rotatable between locked and unlocked breech positions; a tubular barrel extension having a front end coupled to the rear breech end of the barrel and a rear end defining a plurality of bolt locking lugs that rotatably engage the bolt lugs (56a-d) when the bolt is in the locked breech position; and a rear facing annular rim seating surface formed integrally on the barrel extension as a unitary structural part thereof, the rim seating surface extending radially inwards from an interior surface of the barrel extension and arranged to engage a rim of a shell when positioned in the chamber; wherein the rim seating surface is disposed inside an axial cavity formed in the barrel extension between the front and rear ends; wherein the barrel extension includes an inwardly radially extending annular protrusion in the axial cavity of the barrel extension that defines the rear facing annular rim seating surface on a first side of the annular protrusion and a front facing annular mounting surface on a second side of the annular protrusion that engages a mating rear facing annular surface formed on the breach end of the barrel; and wherein the rim seating surface of the barrel extension is circumferentially continuous except for an axially extending extractor slot formed in the seating surface that receives a hooked front end of an extractor that engages the rim of the shell for extracting the shell from the chamber. a longitudinal axis; a receiver; a barrel supported by the receiver and including a front muzzle end, a rear breech end defining a chamber for holding an ammunition shell, and an axial bore extending between the ends; a bolt supported by the receiver for axial forward and rearward movement, the bolt comprising a bolt head including a plurality of bolt lugs (56a-d) extending radially outward from the bolt and rotatable between locked and unlocked breech positions; a tubular barrel extension having a front end coupled to the rear breech end of the barrel and a rear end defining a plurality of bolt locking lugs that rotatably engage the bolt lugs (56a-d) when the bolt is in the locked breech position; and a rear facing annular rim seating surface formed integrally on the barrel extension as a unitary structural part thereof, the rim seating surface extending radially inwards from an interior surface of the barrel extension and arranged to engage a rim of a shell when positioned in the chamber; wherein the rim seating surface is disposed inside an axial cavity formed in the barrel extension between the front and rear ends; wherein the barrel extension includes an inwardly radially extending annular protrusion in the axial cavity of the barrel extension that defines the rear facing annular rim seating surface on a first side of the annular protrusion and a front facing annular mounting surface on a second side of the annular protrusion that engages a mating rear facing annular surface formed on the breach end of the barrel; and wherein the rim seating surface of the barrel extension is circumferentially continuous except for an axially extending extractor slot formed in the seating surface that receives a hooked front end of an extractor that engages the rim of the shell for extracting the shell from the chamber.