A system for firearm barrel modification includes a firearm, a collar and an auxiliary barrel. The firearm has a barrel with holes extending inward from the first muzzle end. The barrel has rifling. The collar surrounds the barrel to serve as a securing anchor for an auxiliary barrel. A sleeve slidably engages onto the firearm from the muzzle. The sleeve contains a tubular component, serving as a barrel extension. The tubular component extends from within the sleeve to form a second muzzle at the opposite end. The mating end of the barrel extension is situated within the sleeve. It butts up against the muzzle and is aligned with the rifling of the gun. Alignment rods protrude from the mating end of the tubular component and insert into corresponding holes at the muzzle end of the firearm. A spring member attached to the sleeve sets in place over the collar.