A firearm comprising: a lower receiver (44,402) having an extension portion with an opening, characterized in that the lower receiver having a mating interface configured to selectively couple with a first upper receiver having a first barrel and a first bolt carrier assembly and a rear lug, the first upper receiver sized to receive a first bullet having a first caliber and with a second upper receiver, having a second barrel and a second bolt carrier assembly, and a rear lug, the second upper receiver sized to receive a second bullet having a second caliber, the second caliber being smaller than the first caliber, an adapter connecting the mating interface and the lower receiver to the second upper receiver; and a buffer assembly common to both the first upper receiver and the second upper receiver and disposed at least partially within the opening, the buffer assembly including a biasing member and a buffer member, the buffer member being movable between a first position when the buffer assembly is compressed, a second position wherein a first portion of the buffer member extends past the opening towards a forward end of the firearm and a third position wherein a face of the first portion remains within the opening, wherein the buffer member is movable between the first position and the second position when the first upper receiver is coupled to the lower receiver and wherein the buffer member is movable between the first position and the third position when the second upper receiver is coupled to the lower receiver, a retractable buffer detent which is held down by upper receiver 356 rear lug 358 when the upper receiver is in the closed position, which when deployed, retains buffer system substantially flush at the extension tube rear end upper surface allowing upper receiver to be removed from and disengaged from the lower receiver. a lower receiver (44,402) having an extension portion with an opening, characterized in that the lower receiver having a mating interface configured to selectively couple with a first upper receiver having a first barrel and a first bolt carrier assembly and a rear lug, the first upper receiver sized to receive a first bullet having a first caliber and with a second upper receiver, having a second barrel and a second bolt carrier assembly, and a rear lug, the second upper receiver sized to receive a second bullet having a second caliber, the second caliber being smaller than the first caliber, an adapter connecting the mating interface and the lower receiver to the second upper receiver; and a buffer assembly common to both the first upper receiver and the second upper receiver and disposed at least partially within the opening, the buffer assembly including a biasing member and a buffer member, the buffer member being movable between a first position when the buffer assembly is compressed, a second position wherein a first portion of the buffer member extends past the opening towards a forward end of the firearm and a third position wherein a face of the first portion remains within the opening, wherein the buffer member is movable between the first position and the second position when the first upper receiver is coupled to the lower receiver and wherein the buffer member is movable between the first position and the third position when the second upper receiver is coupled to the lower receiver, a retractable buffer detent which is held down by upper receiver 356 rear lug 358 when the upper receiver is in the closed position, which when deployed, retains buffer system substantially flush at the extension tube rear end upper surface allowing upper receiver to be removed from and disengaged from the lower receiver.