Lone Wolf now seems to offer AR-15 type receivers and uppers which can be assembled to essentially make a mil-spec quality, AR-15 chambered in .45 GAP. (They also offer these parts for 9 mm). It can be made to accept standard Glock magazines!
This essentially being a hand built gun, it is not cheap.
Why a carbine? It has long been suggested that a semi-auto carbine may be more useful for law enforcement (and perhaps home defense) than a shotgun.
One advantage is that the carbine can be chambered to take the same ammo and magazines used for the duty handgun. It should be accurate to 100 meters, and offer better bullet velocities than the handgun. Magazine capacity could be up to around 30 rounds. It is more accurate than a shotgun, at an expanded range -- which might be an acceptable tradeoff to diminished stopping power at shorter distance in the majority of scenarios.
Such carbines are particularly useful for anti-terrorist guard duty (for example at airports and other vulnerable sites).
An superb example is the Steyr AUG chambered in 9 mm:
It is interesting to note that American law enforcement loves shotguns, while in Europe, semi-auto carbines are much more popular for similar applications.