Congrats on your new gun.
Here is what the H&R website says about additional barrels.
Additional barrels may be added to firearms of our manufacture since 1987 only, with a serial number that begins with N or H. We do not install additional barrels on any other manufacturerÂ’s firearms.
Additional barrels are available as additional barrels only. We will not exchange barrels for existing barrels on shotguns and rifles, as it will change the model.
Rifle frames may be fitted with additional rifle, shotgun, and muzzleloader barrels.
Shotgun frames may be fitted with additional shotgun and muzzleloader barrels only.
hope this helps