The Kel-Tec Sub 2000
The Sub 2000 is a cool little gun because its chambered in 9mm or .40S&W and uses pistol magazines. Its available in several different models that accept different pistol magazines including the Beretta 92mags and the Glock 9mm magazines. This making it the perfect companion for somebody who is already carrying a pistol that uses these magazines.
Imagine this scenario in your bug out bag you put together a Glock 19 or 26 and one of these rifles. When paired with a couple of glock 17 magazines or the glock 18 33round magazines you now have a full set up for a pistol and a rifle that not only share ammo but share magazines.
While your not going to get full rifle ballistics because your still working with the same ammo you will get a longer sight radius, a longer barrel and the ability to put an optic or a rails system on it.
The street price on these is normally $275-$350 so they are even less expensive than the pistol your going to pair them with.