Good Lord! How long has this been going on? Forget this shooting nonsense and bung a couple of slabs of "Ratimor" in the composter (obviously making sure that birds and other beasts cannot get access to it - before someone else jumps in to point out the blindingly-obvious). The rat(s) will cart it off back to their "lair" and in a short space of time drift into gentle and permanent slumber, with no corpses left out in the open. Quick and easy. I reckon there's a few pest controllers on here who could supply said material?