A system that converts an original equipment (OEM) AR-15 rifle to a rifle capable of firing cartridges of at least two different calibers. Two or more barrel liners are provided for cartridges of differing calibers. Each liner is insertable through a rear opening of an upper receiver portion of the rifle and into a smooth bore barrel of a modified barrel assembly. Each liner comprises a tube portion having a longitudinal bore and a head portion with an opening that communicates with said bore. An adapter bolt carrier assembly is substituted for the OEM AR-15 bolt carrier assembly. The substituted assembly includes a bolt that carries the firing pin and is slidably mounted on a rail guide. A front end of the rail guide has a tongue extension with a slot that receives a pin attached to a rear end of a liner head. A support rail is fixed to a rear end of the rail guide. A spring extends from the support rail to the bolt, thereby pressing the bolt against the liner head. A magazine insert, insertable into an OEM ammunition magazine, accepts cartridges of differing calibers.