While Defence is open to acquiring a completely new weapon design, its preference is for modifications to the existing carbine including a shortened upper receiver that is compatible with the M4A1 Carbine lower receiver.
Such modifications are readily available.
Developed for US special operations forces, the Close Quarters Battle Receiver (CQBR) is a replacement upper receiver for the M4A1 Carbine.
The CQBR upgrades the M4 with a barrel 10.3 in (260 mm) long, the modern equivalent of the Colt Commando short-barrel M16 variants of the past.
The purpose of the CQBR is to provide operators with a weapon of submachine gun size, but firing a rifle cartridge, for scenarios such as VIP protection, urban warfare, and other close quarters battle (CQB) situations.