Well you have to remember those at the bottom of the hive are very much those that time has forgotten. They were an outlanders gang, so outlaws. They didn't have the access to steady supply streams from up hive houses. They had autoguns, lasguns, shotguns etc as well. It was mainly the juves that had the lesser weapons.
I quite like that they were improvised. Much in the same way as scavvies who had the speargun along with blunderbusses and other less reliable weapons.
It was all about flavour.