Because people can't fire 40mm grenades from multiple types of weapons? Or larger weapons, if the weapon is built right? For Super Warhawk-sized payloads it would basically be a mini recoilless rifle, but it is still a possibility. More-so for the Light Pistol. Just give them some kind of snub-nose revolver; their version of an MGL-32.
That.
Plus with the nanotechnology and miniaturization processes available at SR lvl technology, i don't see this as being very difficult. Expensive, sure but difficult no. Unresisted recoil should be more difficult for pixies sure, but Again with SR tech, accounting for recoil is not that big an issue, gas venting, gyro Mount...etc.
So in essence i would go with up to and including at least a heavy pistol could be made into a "Pixie rifle/ assault canon" however the ammunition should not be changed in order to preserve DV, so i would limit clip size have all unresisted recoil Count at least double but more likely x3 or x4. So doable, but very very expensive as everything ( gun, clip, recoil sytems...) needs to be custom made.
But honestly this thread and the ones before it about Pixies essentially boils Down to "Some people like pixies, some people are ok with them and some people really really don't like pixies", but thats the beauty of Shadowrun, we can all adjust our own games as we see fit.
And yeah the flying and dumping grenades idea is awesome.