theres a reason I didn't mention the kerans

, its a fantastic starting weapon and is great until your around 50, at which point you realize that its damage just inst going up per ql enough and that your getting enough inits to make the stronger guns (chem/manex etc) to the same rate of fire, at high levels the kerans is just a sense buff gun. And in a similar sense the good days are good low level weapons but there damage doesnt go up enough to keep them in the running.
This is my list for top RK weapons for your fixer:
2x Blinded Blackbirds: Great damage, fast burst (easy 9 second burst)
Manex Catastrophe December: Silly damage, but takes some inits to get it fast and of course you will never manage a 9 second burst as a froob, but more of a 30-40 second burst that will do massive damage
2x Illicit Laser Backups Intra Edition: Again good damage, fast speed and a nice burst recharge, nice mods on the guns aswell, only downside is the pistol skill required to get them on.
Chemical Mausser Streamer: Good damage, good burst, easy to get. Only down side is no modifiers on the gun, hence it gets put low down in the list.
Sol Chironis System: Would suggest this as a last possibility, its fast, but at high level thats a moot point really, its got a reasonable burst cycle although not as good as most of the others, and its damage is fair, if a bit lacking compared to the ones above its barely better then a kerans or russian good day so they could be good alternatives aswell if you have to resort to these measures because you can't obtain any of the above