Provided is an interchangeable modular chamber system for a firearm. The system includes a firearm receiver, a barrel comprising lands and grooves, a chamber insert sleeve comprising lands and grooves, an index pin, a locking collar, and a recoil lug. The barrel is threaded into the receiver with the chamber insert sleeve positioned within the barrel. The chamber insert sleeve is indexed to align the lands and grooves of the chamber insert sleeve with those of the barrel. The locking collar and recoil lug are threaded onto the external threads of the barrel and are adjustable to optimize the headspace for different cartridges of the same caliber and to allow for the sacrificial replacement of the throat area of a barrel.

Provided is an interchangeable modular chamber system for a firearm. The system includes a firearm receiver, a barrel comprising lands and grooves, a chamber insert sleeve comprising lands and grooves, an index pin, a locking collar, and a recoil lug. The barrel is threaded into the receiver with the chamber insert sleeve positioned within the barrel. The chamber insert sleeve is indexed to align the lands and grooves of the chamber insert sleeve with those of the barrel. The locking collar and recoil lug are threaded onto the external threads of the barrel and are adjustable to optimize the headspace for different cartridges of the same caliber and to allow for the sacrificial replacement of the throat area of a barrel.

A rifled barrel has a cylindrical channel longitudinally extending within the rifled barrel between an oppositely oriented muzzle opening and a breach opening, the cylindrical channel having a rifled portion and a non-rifled portion, the rifled portion having a land and a groove helically extending along a surface of the cylindrical channel from the muzzle opening toward the breach opening, the non-rifled portion being smooth and having a consistent diameter along a length. The diameter of the non-rifled portion of the cylindrical channel is equal to a diameter of the rifled portion measured between oppositely oriented sections of the groove. A transitional section of the rifled portion has a diameter measured between oppositely oriented land sections that gradually decreases along a section of the rifled portion adjacent to the non-rifled portion until equaling the diameter of the non-rifled portion.

A rifled barrel has a cylindrical channel longitudinally extending within the rifled barrel between an oppositely oriented muzzle opening and a breach opening, the cylindrical channel having a rifled portion and a non-rifled portion, the rifled portion having a land and a groove helically extending along a surface of the cylindrical channel from the muzzle opening toward the breach opening, the non-rifled portion being smooth and having a consistent diameter along a length. The diameter of the non-rifled portion of the cylindrical channel is equal to a diameter of the rifled portion measured between oppositely oriented sections of the groove. A transitional section of the rifled portion has a diameter measured between oppositely oriented land sections that gradually decreases along a section of the rifled portion adjacent to the non-rifled portion until equaling the diameter of the non-rifled portion.

A machine gun has a barrel, a receiver assembly comprising a cover, a receiver base and a butt; lock frame comprising an extractor and a gas piston; bolt, return mainspring with guiding bar, gas piston tube, flash absorber, trigger mechanism, gunsight, butt sighting appliances, trigger pull and loading case. A barrel is made of a preform produced of steel doped with chromium, molybdenum and vanadium or chromium, nickel, and molybdenum, or chromium, molybdenum, vanadium and nickel. The barrel is coated inside and steel comprises the additional chemical elements.

A machine gun has a barrel, a receiver assembly comprising a cover, a receiver base and a butt; lock frame comprising an extractor and a gas piston; bolt, return mainspring with guiding bar, gas piston tube, flash absorber, trigger mechanism, gunsight, butt sighting appliances, trigger pull and loading case. A barrel is made of a preform produced of steel doped with chromium, molybdenum and vanadium or chromium, nickel, and molybdenum, or chromium, molybdenum, vanadium and nickel. The barrel is coated inside and steel comprises the additional chemical elements.

A modular barrel system for firearm includes a first barrel segment and a second barrel segment. Each barrel segment comprises a tubular member having an axial bore, a proximal end, and a distal end. The proximal end of the first barrel segment has a first threaded element for removably attaching the first barrel segment to a firearm. The distal end of the first barrel segment has a second threaded element and the proximal end of the second barrel segment has a third threaded element which is complementary with the second threaded element, the third threaded element for removably attaching the second barrel segment to the first barrel segment. In a further aspect, method for the manufacture of a modular barrel system are provided.

A rifled barrel has a cylindrical channel longitudinally extending within the rifled barrel between an oppositely oriented muzzle opening and a breach opening, the cylindrical channel having a rifled portion and a non-rifled portion, the rifled portion having a land and a groove helically extending along a surface of the cylindrical channel from the muzzle opening toward the breach opening, the non-rifled portion being smooth and having a consistent diameter along a length. The diameter of the non-rifled portion of the cylindrical channel is equal to a diameter of the rifled portion measured between oppositely oriented sections of the groove. A transitional section of the rifled portion has a diameter measured between oppositely oriented land sections that gradually decreases along a section of the rifled portion adjacent to the non-rifled portion until equaling the diameter of the non-rifled portion.

A rifled barrel has a cylindrical channel longitudinally extending within the rifled barrel between an oppositely oriented muzzle opening and a breach opening, the cylindrical channel having a rifled portion and a non-rifled portion, the rifled portion having a land and a groove helically extending along a surface of the cylindrical channel from the muzzle opening toward the breach opening, the non-rifled portion being smooth and having a consistent diameter along a length. The diameter of the non-rifled portion of the cylindrical channel is equal to a diameter of the rifled portion measured between oppositely oriented sections of the groove. A transitional section of the rifled portion has a diameter measured between oppositely oriented land sections that gradually decreases along a section of the rifled portion adjacent to the non-rifled portion until equaling the diameter of the non-rifled portion.

A projectile for loading into a rifled barrel includes a casing having a cylindrical body, a forward aerodynamic end, and a free end opposite the aerodynamic end, the cylindrical body of the casing defining an interior cavity extending to and in open communication with the free end. The projectile also has a bullet sized to initially slidably engage the casing along a partial length of the interior cavity through the free end. Upon discharge of propellant, the bullet is forced to slidably engage the casing farther within the cavity, preferably such that an entire length of the bullet is housed within the casing cavity, whereby a circumference of the casing is increased such that the increased circumference of the casing catches barrel rifling of the muzzleloader rifle. This allows a smaller caliber bullet to be used in a fouled barrel, while increasing accuracy of the muzzleloader rifle.