I often found air rifles to be much more accurate that air pistols for ratting purposes. You can get assault rifle lookalikes that are pre-charged with C02 and have thirty round magazines (single shot only, not auto). Mind you I wouldn't want to be caught waving one of these around as there'd be armed response officers soon kicking your door in.
I had a cheap Chinese made .177 air rifle until recently that I paid thirty quid brand new for from a stall at a county show. Nasty badly finished wooden stock, break barrel action, but reasonably accurate. A good ratting gun but damned slow to reload. I'd use Racumin poison. It does the job and Ratty loves the taste.